I chose to use the original SM420 4 speed transmission with my V8 transplant. That meant that I could not space the 350 further forward like most people do when they switch to the easier automatic transmission setup. And that meant that I needed to clearance the firewall in three locations; where the rear of the two valve covers are, and where the HEI distributor in the middle is. Also, the valve covers I chose were more rounded, and not so square. That minimized how much clearancing I had to do.
I also had to space out the steering gearbox about an inch outboard to allow room for the wider V8. It still allowed me to use the stock steering system, but if you change over to an independent front suspension instead of the solid axle, I would recommend that. It's just that there is so much less available for 3/4 ton trucks than there is for 1/2 tonners.
I recommend a brake upgrade too, because you'll be a lot quicker, and the weak brakes will become a problem.
I started out with a 350, but when that wore out, I rebuilt a 283 this time for my truck. With a Rochester Quadrajet, and a Dana 60 with 3.50 gears, I get an honest 20 mpg on the highway. Plus the 283 has roughly 3 times the power of the old stovebolt motor.
