I need some advise

Hello Gentlemen
I have just completed a swap in my 72 D100 longbed. I replaced the original 3 speed manual trans with an A833 overdrive. Durring this I had to replace the bellhousing and fiywheel along with a new clutch kit and replaced the 2 piece driveshaft with a one piece unit.
After the installation The truck has picked up a serious vibration that starts around 40 miles an hour and goes thru 65 or so. It seems to get a little less after 60. It doesn't change depending on the gear you are in or if you are at a steady speed or if its still in a pull .It does matter if you are in neutral or if the clutch is in or out.
I took the drive shaft to a local drive line specialist and had it balanced and new joints installed. Still remains the same.
I have noticed that if you keep the truck in a constant pull thru the gears it seems to not vibrate until you let the suspension settle down and maintain a cruising speed.
The only thing I can think to replace now is the transmission mount and I plan to change it this weekend.
Can you guys think of anything else that could cause this? The front bearing was also changed in the trans before installation just to be safe also.

Any help will be appreciated. Thanks in advance