There is a shop in Marietta, GA that will do the swap. With a 5.3L donor (including the pedal and ECM), headers, wire harness, fuel system, mounts, tuning...it'll be around $7-8k. Generally a 5.3L at around 100k miles is an easy find. You CAN use the 700r, but if you have a later model drive by...
They called it "Ms Misery" because it oiled down or crashed at EVERY track it went to. When that thing pulled up, everyone groaned because you were about to get a half hour or more to go get a sandwich while they clean its guts off the track.
I put 08s in my 84 and with a tilt, it's tolerable. The headrests have got to go, though. I really like the extra storage in the armrest. The homemade brackets were a bit of a challenge, but that's where good friends come in handy. Thanks, Mike! I think I like the end result.
I disagree with RecklessWOT's comment; his suggest is NOT the worst that can happen. The worst is you could find yourself being accused of operating a chopshop. Not sure of the laws in your state, but if you are found to be swapping VINs in GA without going thru the legal nightmare of state...
I'll completely switch "gears" here and suggest a switch to 3.73 from that 3.08 differential. While it doesn't address the weak motor, it will make your truck feel more peppy while you decide on your engine/build/stroker/crate/SBC/BBC dilemma. Chances are you're going to change it sooner or...
FYI, my seats came from a '08 Silverado LS, cloth and power adjustment. Took a bit of work fabbing mounts and they still ended up a bit too tall from the floor. I can barely get them far enough back to be comfortable (I'm a 34" inseam) and have to keep the steering wheel tilted up. The headrests...