[dsraven]

yep. 4,2l vortec. good eye.
I started my 57 GMC build with 2 trucks. a 57 GMC long box 3/4 ton stepside farm truck that had possibly rear ended a grain truck at some point, front fenders/hood hit the deck of the grain truck just above the headlights level on the 57, I assume grain truck because the GMC only really spent a couple of years on the road before it became the farm truck full time. I ended up cutting that cab apart because it was tweaked and the doors wouldn't fit properly after I rehinged them. the openings weren't right to fit the doors. the other donor was a 58 Chev 1/2 ton long box fleetside. both were 235, 3 on the tree trucks. the chevy had a hard life and the fleetside was caved in on one side and quite badly rusted everywhere. the GMC ran, it actually started in -30 with just a boost. not plugged in. I drove it up onto a Uhaul deck trailer under it's own power. no brakes to speak of but it ran. the 58 was also a farm truck but it wasn't running, however was not siezed, I just didn't spend any time trying. I sold the drivelines from both because I planned a 4.8l LS engine with auto trans. I used the frame from the 58 because the 57 had been a hydraulic deck truck and the frame was butchered. the 58 frame got cleaned, blasted, boxed, etc and a TCI MII front end was welded in. complete with airbags, p/s etc. it was an ordeal because the cross member turned out to be for a different truck than mine so it didn't fit the frame well, lots of air gaps between the parts. like 1/2" airgaps . I tried to get some dimensional and geometry info from TCI but it was real vague. the supplier I bought it from was just that, a supplier. no help there either. since I am a journeyman mechanic before becoming a Firefighter I figured out the geometry and made it work properly but wasn't impressed. one day I was doing some work on my nephews chevy trailblazer and thought the under side would fit under my old truck. that would give me bigger brakes, rear air ride if I wanted it or stick with the coils, 4 link rear, posi with decent gear ratio (3.73), decent park brake set up with a floor mounted stock brake lever, a steering column with tilt and wiper switches etc, a decent set of pedals with drive by wire accel pedal built in, rack and pinion steering, strut front suspension with lowering kits available. auto with O/D, fuel injection with a fuel tank etc already in place, complete exhaust system (for now but will upgrade later), a complete driveline so no goofing around getting driveshafts built, all wheel drive and an inline 6 that has comparable torque and hp to the 4.8 that I was going to use in the first place. besides, it has some "what the heck" factor as well because you don't see too many customs with an inline 6. long story short I was looking at doing that when another member on here started a project doing that same thing. except with a v8. we did some chatting back and forth and I found a 2004 GMC envoy at an insurance lot for cheap and the project took a turn down a rabbit trail.
SO, you are right. 4.2l Atlas vortec under my hood. if it turns out to have not enough juice to pull my boat out of the water then I will turbo it or something, lol. worst case scenario the envoy did come with a v8 option that could easily be adapted into my truck.
my 57 uses the complete envoy undercarriage and engine/driveline. the frame is modified ahead of the engine to accept the stock bumper. I used the floor and firewall from the envoy and grafted that into my 57 cab since it was quite rusty on the lower 1/2 including the floor which needed to be replaced anyway. the envoy was an XUV, which was a different unit that was longer wheelbase and had a sliding rear roof and a midgate. kinda a cool idea but not something everybody wants. I used the frame as is without shortening it because I wanted to extend the cab to fit high back buckets with integrated seatbelts. the wheelbase was just right. another envoy bonus especially if I use the envoy floor, console, carpet etc. off the shelf purchases when the time comes to do the carpet if I don't like the look of the original carpet. this will be a daily driver so not looking for perfection. I will use the envoy heater as well because it fits right into the firewall and will be an easy fix to get the A/C working etc. again, off the shelf parts usually in stock at a local auto parts place.
so, now you know.
here are some pics of the before trucks and sorta what it looks like with the envoy under it. newer style box with old fenders bolted on.