I have a vibration that starts around 50, gets real bad around 55-60, then goes away. Doesn't do it with the clutch in. U-joints are new, checked that they are centered in the yokes. No noticeable slop in trans yoke, has new bushing and oil seal. Checked pinion angle when it first started and it was OK, don't remember the numbers now. Had driveshaft balanced. Also checked the pinion was tight.

Just replaced the tranny mount tonight. The old mount wasn't real bad but it was missing the nut that holds it to the mount, basically the trans could lift off the mount. I'm guessing this could cause the vibration? Car is a 70 bee with 4 speed.

Bad trans mount caused a similar vibration in my GTS convertible years ago...

High frequency or low frequency vibration? is it vibrating real fast at 3K engine RPMS? or is it lower frequency similar to a tire? Tim

well, if the vibration went away with the new mount then you have your answer, did you do a test drive?

Is tailshaft busing origional or has it been replaced and is flywheel origional to motor ?

It's been awhile since I drove the car (it's undergoing a minor "freshening"). The vibration is speed sensitive, feels the same as a bad u-joint. Haven't been able to test drive it yes since the front clip is still off. Tailshaft bushing is new. Flywheel has been the same for 15 years. Engine is a 400 stroker with a 4 speed. The last few years the vibrations have gotten much worse so I don't think it would be the flywheel. Also swapped to a different set of tires with no change. Hoping it's the mount but it will be a few weeks, hopefully, before it's back on the road.

Rotate your tires if you can or get them balanced.

I chased a similar problem with my Challenger.
I too had a balanced driveshaft, or so I thought.
First d-shaft was a cut-down and balanced one. Finally had it checked by another shop...yep, out of balance. Had a brand new one built and balanced by them...yep out of balance...

Had a third d-shaft shop rebalance the second one, and the problem was finally solved.

Driveshaft was my other thought. When I had it balanced they said it was "pretty good". I was going to have Denny's do it but they said their balancer wouldn't work with the inertia ring on my shaft. might be easier just to have a new shaft built, sans the ring.

Put it up on jackstands and run it in 2,3 and 4th gear to say 4000 engine RPMs, if it shows up in high gear only then it is probally the driveshaft, can you have someone watch it from under the car (a safe distance away from the shaft in case it comes out) to see if it wobbles or acts up some other way

Since it changes when you push the clutch in I would doubt it is the driveshaft. Many years ago I had a similar situation and I found that the pilot bushing had backed out of the crankshaft so the input shaft and clutch disc where not being held on center, making the assembly out of balance.