The other responses are probably good advice, but as I read them I thought about a truck I saw this past weekend at the Canby Swap meet.
It was a 3800, so not quite as big as what you are dealing with, but it had a new rear with modern brakes, and a 350 under the hood. Flat bed with duals in the rear.
Transmission was an SM420 and the front axle & brakes looked stock.
This thing was for sale, so a little skepticism might be order, but the owner claimed it would drive down the freeway at 65 just fine, and that it was fun and easy to drive.
At the Puyallip Good Guys show in July there were two trucks sitting side by side. A 3800 and a 4400 both with 350s. Both owners seemed to think their mildly hot-rodded trucks were great to drive. The 4400 looked completely stock unless you crawled under it, or popped the hood.
So what am I trying to say? I think if you want to hot rod a big bolt you should go ahead and have fun. Just keep in mind that it won't be as easy as rodding a half-ton...
