Pulsing vibration

[lolcopter]

I've had a pulsing vibration out of my 68 Gmc since I've gotten it. The vibration starts at 60 peaks at 65 and is gone at 70.

I've had 2 different engines (350&454) and transmissions (turbo 350 and a 700r4), 2 driveshafts both rebuilt prior to install, rear end rebuilt with different carrier, new rims and tires, and redid all brakes. I can't figure out what is causing vibration.

The vibration has stayed constant regardless of configurations the truck has been in.

Any help would be appreciated.