driveshaft shortening

It's easy to SHORTEN a driveshaft. Only shorten it in the back and only on a driveshaft that has enough non-tapered area to work with. First mark it so you can weld it back together with the same orientation. Cut it off to the length you want right thru the back weld, then tap the rear yoke onto it, Install it into the car with the rearend on stands; not the body. Mount a dial indicator up to it and gently turn the tires to turn the driveshaft; careful that you don't jar against the indicator. Tack weld it in three places; 120 degrees apart. Double check with indicator, then remove and weld. Weld any balance weights back on where they were. You marked their location didn't you? As long as you weld it centered; even if it comes out unbalanced; which it probably won't, you can take it in and have it balanced pretty cheaply. I've done quite a few this way and have had zero balance problems.