iv just built my 66mustang and when i took it for a test drive iv been feeling this vibration that iv had in the previous motor also. the motor and all the parts were blalanced at a shop and as far as i know all the rims and tires are ok because i balanced them myself at a shop i work at and the problem is when im over the 45mph mark i get this vibration throughout the whole car and it never goes away unless im under 45mph. the faster i go the faster the vibration gets, the vibration isnt constant, its more like a rolling kind of vibration as if it gets loud and goes away, gets loud and goes away, and its driving me crazy because i dont know what it could be, when u let off the gas its almost constant when ur at high speeds. when i throw it into neutral the vibration is still there but its not as bad. it also occurs at 2000 rpm or higher no matter what gear its in, it feels like the faster the engine is spinning the faster the vibration and it also has a vibration when in park and reving it at 2000 rpm anything above 2400 isnt felt anymore, only when its in gear and moving down the road.. somone please help me if you have a good idea of what it could be.. thank u so much in advanced!!!  

Things to consider:
Driveshaft out of balance or out of phase
Bad u-joint
Wrong flywheel or flex plate
Converter ot clutch problem
Damper ring shifted

You might explain a little about "this motor" vs "previous motor".
Sounds like you work at a shop - check the u-joints first, then put the car on a lift and have someone "drive" it at various speeds while you observe the driveline.

When the shop balanced the engine, did that include the damper and flywheel used in the car right now?  

My first thought was an old double wall driveshaft myself, but if you say it vibrates in "park" when you simply feed the RPM in, then it's an engine or converter balance problem.

What exactly did the machine shop balance? Just the crank and rods? Or did they just equal the weights out on the piston/rod combos?

What damper is being used? OE Ford or aftermarket? Any PN or markings on the flexplate for us to possibly ID it?

HTH...
Mark