The 3.50 gears won't work on the carrier for the 2.56 gears without using a ring gear spacer, or using a 3-series differential carrier.
Can't tell you what rpm you will be running without knowing the actual tire size and what rpm you run now with the 2.56 rear @ 60 mph.
You can keep the drive shaft with the 200R4, it is the same. Most racing trans builders say they can beef the 200R4 up to handle more, guys are running them in low 8-second 1/4 mile Turbo Buick Grand Nationals. But, 200R or 700R, the cost really goes up sharply to make them handle 450 or more HP.