Unless you have a good reason, I would just leave well enough alone. These trucks don't go fast or stop fast to begin with. The parts needed to make them do either limit their use a big trucks. Very few have the original engine left in them not that it matters, they aren't corvettes.
I can understand wanting more power to haul a load and am doing a diesel swap (originally intended a v8 swap), but that truck came to me without a drivetrain.
If it goes down the road and stops, a big truck is probably best left alone when it comes to swapping the engine. Otherwise you have a bunch of useless gearing behind that v8.
My 39 1.5 ton with a 55 235 and stock crash box can move my 8,600 lbs boat down the road without a problem and pull it to a reasonable stop from 55 mph (trailer is equipped with surge disc brakes).
just my $.02
