Classic/Antique Car Repair: 1969 Lincoln Mark III, lincoln mark iii, motor mounts

Question
I have a 69 Mark III that has a rebuilt 460, V-8 engine.  I've put roughly 700 miles on the new engine.  The car runs fine at first, but once the engine warms up (about 20 minutes), when starting from stop and slowly accelerating, the car shakes until you get up to a higher speed or floor the accelerator.  If you're already at 55 mph and accelerate, no shaking.  What are your thoughts?

Answer
Hi Dan:
My first thoughts are the "Motor Mounts". Were new motor mounts installed? And were they installed correctly. Also, the universal can also cause this problem, if it is out of balance. If the universal was not removed from the transmission during the rebuild process, the joints may be worn and out of balance. Although bad motors mounts can cause a slight tremble, shaking usually means universal spinning out of balance.