Depends on the transmission used. Most TH350's and all 700R4's and 2004R's shound not require driveshaft mods. The yoke from the 'glide uses the same spline so that will work too.
Some TH350's and all the TH400's are longer and would require shortening of the shaft. In addition, most TH400's use a larger yoke with a larger U-joint. (some very early Buicks in the mid 60's were an exception to the larger yoke)
On both the TH400 and the 2004R, the crossmember will have to be moved back about 6 inches. Some 700R4's have the transmission mount on the main case, not the tailshaft housing. You will need the tailshaft mount to use the same mounting location as the 'glide.
If you go with the 700R4 and the one you have doesn't have the tailshaft housing mount, you can change the tailshaft housing. When doing conversions, I would buy the correct tailshaft housing from the dealer for about 35 bucks.
For informational purposes, one of the OD (700R4 or 2004R)transmissions would be the best. Both have lower first gears than any of the three speeds, an overdrive gear (remember, the V6 has a torque peak RPM of 1600 and would get the best economy cruising at that RPM on the highway) and the lockup converters take converter slippage out of the equation.
Powerglide: 1.76:1 first gear
TH350: 2.52;1 first gear
TH400: 2.48:1 first gear
700R4: 3.06:1 first gear, .70:1 OD
2004R: 2.77:1 first gear, .67:1 OD
