Let's go back to the top for a moment and see if we can simplify this for Rob.
1. If your trans is a 1964 Powerglide then you will have to use a TH350 27 spline yoke. This yoke also fits all the fully synchromesh 3 and 4 speeds except the M22 Muncies and TH400 automatics. It is an on the shelf item at any driveshaft shop and also by mail order. There is an early style with the retaining snap rings on the outside of the bearing cups and a later style that has them inside, otherwise they are identical. Get the one that matches the yoke welded on your driveshaft. Should be the early style external snap rings.
2. A TH350 with a 6 inch tailshaft is for all our purposes the same length from the back of the block to the rear seal as any of the fully synchro Muncie 3 or 4 speeds and Saginaw 3 or 4 speeds when also measured from the back of the block to their rear seals while attached to the bellhousing. All have the mount the same distance from the back of the block as does the P'Glide.
3. The P'Glide is about 2 inches + - longer overall than the TH350 or standard shift set ups and some driveshaft work will be needed.
What Gordy points out is correct for changing to a short tailshaft. TH350's are almost a dime a dozen and there are tons of 6 inch units out there. Might not be worth the trouble and expense to have a shop do the work. Usually they will want to go through the whole trans if they are going to have to stand behind it. If you have done auto trans rebuillds then you should be able to handle it yourself.
