If I mod a longtail driveshaft housing by adding an upper and lower bearing housing kit it might shorten the driveshaft housing some. I will have to cut off the thin part on each end of the housing where it was machined for bushings and seals to have enough good metal to weld the slip on bearing housings.
Is there a Hoyle's rule for housing to driveshaft length, or will I be ok? I will have to cut 3 3/8" off of each end. The bearing housings will give back some of the length but I don't know how much. Driveshaft is 6'
Whatcha think?

Sounds like you got it under control, just be sure to maintain the original length to allow the machined part of the shaft to work for each end. Id cut one side first, tack, measure down and mark, cut, tack, take a final measurement to be sure the length is the same as original, and weld...