Aside from that, in addition to motor mounts, have you tried the
transmission mounts? Center support?
Before you go for a complete new drive shaft let me throw out one more
option. I was in pretty much the exact same situation a few years ago
with my '76 Spider. I'd replaced the center support because it was
clearly shot. I also installed new u-joints and donut at the same
time. There was a persistent vibration at a particular speed that I
simply could not diagnose. I was about to go for a complete new shaft
when a friend suggested one more thing. He said to check the tightness
of the large bolt that holds the mounting hardware to the transmission.
This is the large bolt that holds the piece onto the transmission to
which the donut is attached. I removed the drive shaft and obtained a
large socket wrench suitable for this nut. It seemed very tight with
no noticeable play but I put a wrench on it and torqued it down maybe a
third of a turn further at the most. Silky smooth drive shaft ever
since.