If you're truly going to use the '64 833, have a driveshaft built with a rear u-joint that matches the pinion flange on your 8 3/4 rear, and get the ball and trunnion elimination kit so you can use a standard u-joint in the front also.....hopefully one that matches the rear for simplicity. The other issue you will run into doing that conversion is choosing and using a bell housing/clutch setup that will fit properly. A stock '64-6 273 cast iron bell housing, flywheel and dust cover will work great if you also use a B-van 6 cyl clutch and heavy duty pressure plate. If you go with an aluminum bell housing and matching flywheel/clutch/dust covers, you will want to make sure the bearing retainer hole matches the small bearing retainer on your 833. You will also most likely need to modify your clutch linkage for either setup if you plan on re-using your 6 cyl. setup. As for your question as to whether you "can" use your old driveshaft, if the rear u-joint matches the pinion flange on the 8 3/4 (or you get the conversion joint), and you're willing to shorten the shaft a bit to account for the 8 3/4's larger size over the original 7 1/4", sure, it will fit. Not a great idea doing that. Even though Max wedges and Hemi cars had the ball and trunnion joints, they were the bigger version than what your '64 6cyl 7 1/4 uses. With 450 HP, it would be a grenade with the pin pulled IMO.
