Vintage cars do not transfer drivetrain vibrations like many are describing above unless there is an issue.
You need to look at the engine mounts and the transmission mount. Sacked out mounts transfer a lot more vibration to the chassie. Another big culprit is the exhuast dead up agaist the frame, a torsion bar, the steering column etc; look for anything that is making solid contact.

My trans mount resently failed in my GTX when I was out thrashing on it pretty hard and it had a bad vibration at different rpm ranges in nuetral and a drone at highway speed (sounded like a out of balance drive shaft) dropped in a new poly mount and it's quiet and smooth again.