Here's what I'd do (with caveats); find a WCT5 with a .72 or so 5th gear. It's an easy installation and would be the perfect transmission for my use. However, I don't do high rpm clutch dumps or other abusive driving in my truck (at least in a hot rod/race, fast truck sorta way). 1st gear would likely be in the high 3s making it super easy to get underway and 5th would make for a decent freeway cruiser even with the 4.57s in your truck if the rear is original. A hard to find set of 4.11s would be a good upgrade since you'd have plenty of power to pull the higher gears in 5th.
Now if you're planning to use the truck as originally designed and put it too work pulling loaded trailers a T5 "may" be on the light side.
Lugnutz is a GREAT source of info related to all things T5 and he's doing a V8 drivetrain with a T5 currently (may have completed it by now).
Good Luck,
RonR
