I was going to perform a similar conversion on my 70 gmc and decided to back off of it for now. What I learned is that the exhaust won't fit in the frame without either cutting the frame, or finding another exhaust setup. Headers or corvette manifolds will work. The A/C compressor will need to be swapped out as it will not work with the older expansion valve setup. Naturally, you'll have to install a fuel pump, and rig some sort of spedo cable if you are going to use the original gauges.

I'm looking forward to seeing what you come up with. My frame-off budget just wouldn't let me do the full 5.3L upgrade, so I'm going to put my 383 back in with a 200R4. At least this way I'll have overdrive.

Oh yeah, I forgot to say that for the newer transmission to fit, you'll have to drill new tranny crossmember holes in the frame. They will need to be 1.75" back from the 350 turbo mounting holes.