In terms of acoustics/vibration analysis, there's more than one way to
do it...  

The solid Tyranny mount is the source of the noise, but the panels
(floor/side/ceiling) are the transducers, i.e., they are what create
the sound through their vibrations.  In case that you cannot find a
good replacement for the solid mount, the other course would be to go
over the bus, find whatever panels or parts of panels are 'undampened'
and 'dampen' them (acoustically speaking)...  Carpeting wherever
possible does the trick for most.  Undercoating also helps a lot.
You might try to somehow put some insulation inside of the rear
quarter panels.  Side panels, top, front doors are all great
transducers.  insulating them also cuts down on noise.  I do not
remember all your postings, but you've already done quite a bit in
this direction, if I'm not mistaken.  One thing to do is to get
someone to drive your bus (auuughh... choke. choke. -- gulp!) while
you're in the middle and press your palm  against the panels from the
inside -- you can identify spots that change the sound level that way.
Those are the areas you want to 'dampen' first.

I used this method to make the two 73 buses (relatively speaking)
quieter back in our student days...

Cetin

 > (snip)
 > 
 > The last big noisemaker I have to take care of is the solid tranny mount.
 > Anyone have any eperience with an early mount conversion piece that wasn't
 > solid?  I'm thinking about buying one and modifying it, cutting a slice out
 > of it an putting in a piece of rubber instead...
 >