[dsraven]

when I did mine at that point (57 gmc on 04 envoy) I trimmed the envoy floor to allow steps. I also used part of the firewall but if doing it again i would use more of the firewall. it has the mount for the brake booster which is also the mount for the pedals so I ended up welding some areas to accept the stock pedal adapter plate.
I placed my cab down over the frame and floor pan with the firewalls lined up and the floor sitting on its original mounts and bolted up. then i used some threaded rod attached to the cab as a way to adjust and fine tune the cab to frame dimensions. I lowered the cab until the body was about the same height as the lower edge of the frame. that way you won't get a surprise one day on a speed bump. the step vertical flanges were shortened accordingly. the problem I now have is the stock envoy seat attached to the floor is a little low for the dash height. i could have shimmed the body off the frame, like a 4x4 lift kit would do, but it is a little late now. maybe I will shim the seats instead. it could go up about an inch and half. you decide on yours because it is your truck, but I thought I would mention that point before it gets too late for you too. using the stock envoy floor and seat arrangement means the seat will be further away from the dash so your should then becomes closer to the rear of the door opening. I suggest to do some tacks when you get it like you like it and then install the seat and see how you like it. the floor pan is rust coated so ensure to grind that off in the areas where you will weld, those fumes really aint good for you. the weld thru primer is good to have kicking around as it is a fairly good rust inhibitor to spray over the welds and it dries fast
pics show envoy suspension at stock ride height. I pinned the suspension prior to removing the body so I would know where it is supposed to sit. the springs can be adjusted afterwards if the body moves a bunch up or down when the suspension pins are removed.the envoy/trailblazer/saab came with air ride as an option on the rear so I could do that with stock air bags and remove the coils since all the frames have the sockets for the bags to mount into. the fronts are a strut so there are lowering struts available if needed up front.
keep posting progress pics, it's interesting stuff. also, keep doing your body dimensional checks to ensure the doors will still fit when done. things get tweaked easily and you don't notice. i cross braced mine but there was a time when the floor, roof, firewall and rear of the cab were missing.I extended my cab so a seat would fit.
you may have noticed the floor hump is not in the middle but offset to the passengers side.