You s/b good with your converter; Moose described exactly how it should look / feel, when you go to join it to the flexplate.
It's probably a good idea to use new bolts to join them together, and not reuse the old ones. If you removed the flexplate from the crank, you *must* use new bolts there. Obviously, you're interested in and taking care to do the job right, so figured I'd mention the bolts.
Good move on your part to measure the distance between converter <> front lip of the bellhousing, before removing it. A lot of people wouldn't have thought to do that.
You mentioned the oil pump seal - yes, I'd replace it, since you're there, as it's cheap insurance. Fel-pro's is well-regarded, as is Melling's (the fel-pro is easier to find at retail.)
If your new engine has more than 80-90K on it, give thought to a pump replacement (although you'll have to pull the balancer, so if that's a concern, you can leave it in place, and just remove the pickup tube to get to the seal.) Do your homework on the proper seal (type and color; GM made revisions!) and when you join the tube back to the pump, you should feel a pretty snug connection between the two. Otherwise, you'll have no oil pressure at startup.
The nuts that hold the pickup tube to the engine are OK to reuse (they're not stressed, or fastening the end caps to the crank). Their light torque spec is indicative of that. While you don't have to remove the windage tray behind it, if you do, you'll get to see the crank & rods in all their glory, if that sort of thing interests you. You can also (mostly) see the condition of the cylinder bores; if they're in good shape, you'll likely see the crosshatch hone marks on the bores - if so, that's a good indication the piston rings are still in good shape; good to know.
If you turn the crank to move the pistons, take out the spark plugs first! Also remember that the crank *always* turns clockwise, as you face the front of the engine & turn it. It can move CCW, but you shouldn't do that.
If you do remove the oil pan, it would be best to join the trans / engine back together beforehand, and reattach the bellhousing cover, before removing the oil pan. The alignment on those matter, as they are both stressed members of the engine, believe it or not.
Finally, remember that the two longer thinner M6 bolts at the back of the pan do NOT get the same torque as the others. If you forget, you'll snap them off. So don't forget... LOL
OK... that's enough for now. Plus, my thumbs are tired. 😫