You’ve Mixed your Missions together just a bit here... so I’m not sure if you have decided to abandon the GM 4.2L Atlas engine for an equally difficult (if not more so) Engine and Transmission swap to the GM 350 CID V8 Motor. Having steady transportation now will take the pressure off of you and the urgency to make a snap decision. In the meantime... if you want to work further with the 4.2L engine... hopefully. before you removed the Crankshaft Bolt and pulled the Harmonic Balancer off of the engine... First you set the #1 Cylinder to Top Dead Centrer. If you are intent upon doing any work on the engine that includes Reconditioning the Engine Head, Figuring out if the Timing Chain Entire apparatus is involved in the catastrophic failure, including the VVT Phaser as suggested by
@Sparky and
@Mooseman, then the best thing to do is to pull the engine head while its still inside the TB without removing ANY of the Camshaft Hardware first.
You will need to secure the two camshafts with their rear exposed “Flats” Horizontal with the Engine Head and facing UP with a special tool...but ONLY do this if you have previously positioned the #1 Cylinder at TDC. If you cannot rotate the Crankshaft Clockwise ONLY to achieve TDC in the #1 Cylinder... you wont be able to do it now without reinstalling the Crankshaft Bolt with NO Harmonic Balancer there... and you risk “pranging” or damaging the “Tuning” of the Crankshaft by turning it in this sketchy manner. But... before going one step further... if you WILL be removing the Front Timing Cover, the above step will not apply and if you are intent on trying to replace the two Timing Chain Sprockets and Chain, reading
@Mooseman 's Epic Post on this job then becomes paramount. After removing the two Timing Chain Sprockets and allowing the Timing chain to either be held fast to prevent it from falling off of the Crankshaft Cog... or with the cover off, completely removing the chain, the next procedure is the removal of all the Head Bolts.
Following a Reverse Torque pattern is described as a “Best Practice with Aluminum Engine Heads” to minimize the chance of warping the head. To minimize Breaking Off head Bolts... This must be done by first Hammering on each Hex Cap Screw (Head Bolts) Dead Centre with a Black Steel H10 Hex Socket inserted and also fitting the H10 Tool with a “Sacrificial” 1/2” Male to 3/8” female adapter to “Beat the Hell Out Of It” and loosen the grip of each bolt. Doing this first lessens the chances of breaking off most of the bolts. You will have to be very careful not to become overzealous and “Miss the Mark” when applying the hammer blows straight down. After the Bolts either unscrew properly or... break off... you will find lifting the head impossible unless you first unscrew the two Plastic Chain Guide Covers and then unbolt the Left and Right Chain Guides to free the head there, too.
Take your time with using a Breaker Bar and use several VERY TINY, VERY SLOW passes when trying to unbolt the Head Bolts. Use several, reasonable Hammering Sessions in between your Breaker Bar small but steady pulls... especially at the BACK OF THE BLOCK. The last place you want to be doing Centre Drilling of Broken Bolt Shanks and using Easy Outs is Under the Cowling at the Firewall when working on Broken Bolts around the # 5 and/or #6 Cylinders. I cannot emphasize this enough... Use the Leverage of your Breaker Bar... Gingerly...and you might save yourself a week’s worth of worry and extra toil trying to extract any Head Bolts that WILL break off if you get too hectic with the tools while trying to get those 14 Bad Boys out. You do NOT want to “REDLINE” your RPM Here... ;>)
I wound up using a Plastic Covered Bicycle Security Chain that I snaked in and around the open spaces of the Intake and Exhaust Camshafts and once chained up ...it lifted straight up with the Engine hoist. The head only weighs about 100 Pounds, but mind you... if you start dragging those soft Aluminum under-surfaces across anything and scratch them up...you will wind up needing another engine head. Use small blocks of 1”X2” Pieces of Wood to slip in between the underside of the head and the Upper Engine Block to rest the head and make support adjustments as needed. It is best to lift it off the engine block VERY SLOWLY and then just guide the lift straight back. Walgreens sells an assortment of Walking Canes that can be very handy when positioned under the hood to hold it up and and out of the way of everything as the extraction occurs.
The best way to deal with that PITA Low Side AC Return Line at the top front of the engine...is to first undo all of the retention fasteners at its hold-down flanges...and then unscrew the clamp that holds the entire Accumulator and Rubber Boot. Then by carefully lifting the Accumulator Up and Out of its position inside of the White Plastic Coolant Reservoir... the short Flex-Line will loosen and give you just enough wiggle room to pull the Aluminum Pipe towards the Radiator and well clear of the upper engine. This will leave you with enough room to R&R the Head and not have to disconnect and bleed out your AC Lines. Use sturdy Zip-Ties to hold the AC towards the front and towards the passenger fender well.. .but do NOT pull too hard ...or you'll wind up kinking the Aluminum Pipe.
If you have to close the Hood later on...be mindful that while in this position...the Accumulator will have to be gently pushed down just enough to close the hood and not damage the AC lines. If it turns out that you have to pull the Engine Block later on... not having the Engine Head blocking access to the two upper, hidden Engine to Transmission Bolts at the top and under the Firewall area makes unbolting the Transmission much easier.
Once the Engine Head is removed from the Block.. "Bench Working" on it to R&R the Camshafts, Rocker Arms and Push Rod Assemblies will be a breeze while it is elevated and resting upon a pre-fab set of clean wooden rectangles that serve to prevent damaging any Open Valves that will be vulnerable underneath and easily harmed if arrangements to support the head when it is sitting face down on the bench have not been anticipated.
As for the Alternative Mission ... The Theoretical Engine Swap issues become Real Problems if you have a 4WD flavour in your Trailblazer... as the GM 350 CID Engine (Stroked to 383 in your case) has no “Hole Through the Crankcase”. Also, the shape and conditions necessary to align the back of the block to the 700R4 Transmission and the need to accommodate the V8 Oil Pan-to-Front differential issues must also be dealt with. It goes without saying that figuring out how to mix the PCM, Power Steering, Alternator and Accessories as well as Engine Cooling Power and Ground Cabling , Throttle Cabling and Accelerator Pedal Requirements and of course... a Carburettor Fed NA GM 350 CID Engine and Exhaust problems must also be solved. However... Here are some GM 4.2L -to- GM 350 CID Engine Swap videos that prove it can be done:
https://www.youtube.com/watch?v=nJ0SueqAtPQ