Before I would start throwing parts at it, I would first figure out if in fact the problem is in the engine.
I would check and adjust the oil levels,both engine and trans to eliminate windage.
I would get the rear wheels off the ground, and in neutral , buzz the engine up into the vibration zone, noting the degree off vibration;
then one atta time;
> check that the pinion angle is in the ballpark and that the u-joints are smooth. see Note-1
> flip the belts off and retest,
> with the engine in the vibratory zone, I would arbitrarily advance/retard the timing on the fly, and note the difference, if any. See note-2
> take off the driveshaft and plug the hole with a spare yoke, then retest. see note-3
> isolate the engine from the trans by unscrewing the convertor bolts and pushing the TC back into the trans and securing it back there.
I would of course stop when/if the vibration goes away; and after each test, I would undo the changes I just made.
Note-1
As regards pinion angle;
if the rear is too far out, or if the front is not right; then you get a vibration as the U-joint angles fail to cancel.
The vibration gets worse on power application as the springs allow the nose to come up.
Vibration may ease on deceleration, as compression-braking pull the nose down.
hen you hit your chosen cruising speed, you ease off the power and the springs push the nose down. When the trans goes into od and L-up, the engine rpm is forced to decrease which decreases the power, allowing the pinion nose to drop again.
The change in pinion angle with soft springs can be over 10 degrees.
The change at the front is only gonna be 1 or maybe 2 degrees, in sync with the rear.
The front angle MUST remain a minimum of about 1 degree.
The driveshaft HAS to be going more downhill to the rear, than the trans, and at the rear, the pinion has to come down to it. In this way, the idea is that at cruising speed the two angles are ideally equal and opposite. But this situation can almost never be created because compensation has to be ADDED for spring wrap-up under power.
The thing to not have happen, ever, is that as the nose wraps up, it over-centers and causes the rear angle to climb higher than the driveshaft centerline. When this happens, the angles add together, causing large amounts of vibration.
Softsprings and/or a lowered car can make this a real challenge to sort out.
Note-2
The Vacuum advance system must be hooked up and WORKING.
don't bother with the timing light, just pull/push the Vcan, until you find a sweetspot; THEN measure it.
Note-3
The yoke has to be in there to center-up the output shaft. The rear of the trans has to be higher than front so it stays in there, at rpm; check it if you run it up more than once.
Note-4
If the vibration disappears when you remove the driveshaft, then everything back there is suspect.
That's what I would do
Good luck
