You need to measure your drivetrain to have the driveshaft shortened properly.
Get the car up high enough to crawl under and measure, wood blocks under each tire works well. The car must be on level ground with all of the weight on the suspension.
Push the driveshaft yoke all the way into the trans until it bottoms, then pull it out by 7/8". Then measure from the center of the U-joint bearing cups on the tranny yoke and pinion yoke. Take this measurement to the driveshaft shop and have the new shaft made to this dimension.
Not all driveshaft yokes are the same length, the same for pinion yokes, plus different rear ends (8.2", 8.5", 12-bolt, etc.) will alter the distance between the bearing cups (driveshaft length). So you really must measure your car to know what you need.
The TH400 crossmember will bolt to the existing holes in the subframe. Typically you'll have to shorten the shaft by about 1" when swapping a TH400 for a TH350, but again you should measure to be sure.
