Warren,
The earlier 28's had a multidisk clutch and the solid front crossmember. I believe it's in the service bulletins that around October/November the solid mount changed to the yoke and a single disk clutch also was a changeover.
Vibration can be attributed a little if you have the solid front mount. See the service bulletins for conversion if you desire... or to restore, or if you wish to keep original just keep the solid mount. Ensure the rear mounts have good rubber in them and not sagging. You might post a few photos of what you have for further input.
FYI.. there's nothing wrong with a multidisc clutch if they are working well. they are very smooth. the only problem is there are no clutch facings available for them and few know what they are doing if indeed they need service.
If you wish to change over to a single plate clutch it is possible and I have done it a number of times. There is a list of parts required to accomplish it that must be secured first.
Larry Shepard