There have been a few guys who have installed the ZF 6spd also which is a good tranny, get the ford version, GM opted out of the OD lubrication pump, ford different.

I bought two used NV5600's one for 1500 one for 1800 i believe when they finally needed to be rebuilt it was a 2k job but they lasted 250k miles of hard 12v pullin and the 3-4 split was perfect with a 12v even with twins especially in the moutains where corners require 3rd gear and the next straight stretch is a serious uphill grade but staying in 3rd tachs you out and direct is gonna bog things down