My suggestion if you want a fully reliable driver is to get an insurance job truck from 2000-2010 or so, with the 6.0 gas and the 4l80 or the 6l90, and swap it. Don't get a half ton, it'll have a 4l60 and those shouldn't exist.
I'm a big believer in fuel injection. I'm a big believer in modern transmissions.
A ly6 with headers, an intake, and a tune will do 425hp. A 6l90 is FAR superior to any th350 known to man.
A donor truck up here with 100,000 miles on it is worth 800 bucks. The engine mounts will be all but the same. The trans mounts will be all but the same. It'll run like a top, it has a huge aftermarket, and it's made of iron.
Just my two cents, because I hate old ass autos and I hate carburetors. The 4l80 is an exception, as it's an evolution of the th400, and that was a decent old auto. Still sucked alotta power out.