I have heard that it is almost easier to drop the old body on a newer truck chassis than it is to update the old chassis for modern engine, suspension, etc.

But considering you want to still use a stovebolt, then the old chassis is what you probably want to stick with. I would think you could still get the front suspension out of a modern truck if it's a 2WD version, and enjoy a much smoother ride without having to totally alter the ride height of your stovebolt truck. That's what I'd like to do someday.

The old suspension is VERY stiff, but it's okay. Why not go ahead with the bigger inline 6 anyway, while you decide what to do with your suspension.

52 GMC 3/4 ton pickup
68 Big Block Vette
68 455 Firebird