The differential is easy enough. Install some 31 spline axles, 31 spline trac-loc and your choice of gear ratio in the rear end you currently have. I would also suggest going with a good aluminum differential cover as well. With a stout 408W, a 3.73 gear is plenty for mixed street and highway driving.
As for the transmission, if you want to go with a standard transmission, its hard to beat the T56. I have one in my 94 Gt with a 393W and its very nice. 2000 rpms at about 85 mph all day long in 6th gear!!! :headbang: