I have the 5.3L in my Envoy, but (I think) I know a little bit about the 4.2L...here goes...
The 4.2L LL8 is a DOHC engine. To top it off, the exhaust cam is hydro-mechanically variable. That's one big reason why the motor has good power throughout its entire RPM range, down low
and up high. It's damn near the equal of the optional 5.3L (which has a wee bit more TQ, mainly benefiting those who tow.) I have a lot of respect for the 4.2L as a result.
If you
could find someone to make a custom grind for you, get ready to open your wallet.
I think Texas Speed and Performance (
www.texas-speed.com) might be a good resource to check with.
PCM of NC (also one of our vendors) offers some packages for the 360s - but none include a cam upgrade.
Besides the cams, you'd need to adjust fuel / spark tables, so you're looking at dyno time, trial & error tunes...see where this would be going?
There are people here that have / are adding turbos to their 4.2L - at least one that I know of was running a ton of boost (it spends time at the strip). They may have some suggestions for you. Check the forum list for the right area to find them in. I know
@Capote is just starting off a turbo project, btw.
Outside of turbos, there's not a lot of aftermarket support for the LL8 - as good as it was, the engine was orphaned when the 360 platform was retired (being an inline six, its applications were limited, mainly due to size constraints.) The Lambda platform that replaced the 360s is FF, as well.
The engine would fit in the full-size pickups & SUVs - but they already have good motors. Since the 4.2L is a bit thirsty (there's only 1-2 mpg difference between it and the optional 5.3L), it doesn't make sense to retrofit it in a Silverado, Tahoe, or Burb.
Depending on how serious you are about hopping up your Voy - you might be better to replace it with a Denali (5.3L standard) or a TBSS (6.0 LS2 plus 4.10 gear plus AWD). Both are available in the shorter wheelbase (SWB).
There's plenty of aftermarket support for the LSx engines. But transplanting a V8 into your current truck is more trouble than it's worth (it's been discussed several times). Easier / cheaper to get another truck, IMO.