The differential choice is probably all up to you. If the truck is subject to heavy duty, or if you are putting a pretty decent engine in it, then you might want to go with a Ford 9 inch, Chevy 12-bolt, or maybe even a Dana 60 if you want ultimate strength. If it's just a street cruiser, not subject to heavy towing or heavy duty, then save some money and grab yourself a Chevy 10-bolt out of either a 70's or 80's F-body. The 80's F-bodies used a cone-type posi, which works extremely well but is non-rebuildable. That just means you buy a new one when the diff is rebuilt. So no biggie.
