[Roadsters] Vibration

[Mark]

  I had the engine trans out of my 70 SRL and noticed the trans mount 
was in dire need of replacement.  I used the Prothane mount as it seemed 
the closest to the stock thickness.  It was a fairly painless 
swap....but now I have this vibration that starts to be noticeable at 
about 40 MPH - it gets smoother as speed goes up but it is still 
noticeable....does not change with the clutch in or the trans out of 
gear and coasting.  It is very apparent for in the 40-55 MPH range.  My 
first reaction was to replace the u-joints, absolutely no change.  So I 
am now wondering if anyone has a stock 5-speed trans mount they could 
measure for thickness?   Mine was so far gone I could not salvage the 
pieces.....!  Or could this be a drive shaft balance problem?

Mark
-Ohio