***The following is from a related post, I added them here because they pertain to the discussion.***
Well I haven't done my tunnel yet because I've been putting off the decision of whether or not to cut down, reweld and use the Miata PPF or to just bolt the tranny and diff to the frame. I understand what the PPF does in the car, it sort of melds the tranny and rear end into one unit. Therefore when it is used the only true chassis mounting points for the entire drivetrain are the two engine mounts up front and the outer flanges of the diff housing in the back. So, if I get rid of the PPF I will have to somehow manage to secure these pieces together. Bolting them down with rubber bushings probably allows too much movement between the two and bolting them solid to the frame is really no different than using solid engine mounts. (since the engine-on soft rubber mounts-is bolted to the tranny) So what to do?
Another issue is passenger space. The rear diff driveshaft plate is offset to the pass side 3/4" and with the PPF around it there is only about 14.5" or so for a passenger seat.
So here's my real question. What happens if I bolt the tranny and diff straight to the frame with no bushings, thereby making the chassis the PPF? Bear in mind the engine bolted to the other end of the tranny is on rubber mounts. Will this put undue stress on the tranny and break it or the bellhousing? Will it shake, rattle and shimmy making my car an unpleasant place to be?
HHHMMMMMMMMMMMMMMMMMMMMM. Lots to think about.
My 2 cents (and no, you can't have change)
I would solidly mount the diff. to the frame, metal to metal (or a delrin or other really hard synthetic material)
I would use rubber or at most polyurathane for the transmission. The u-joints will make up the difference between the transmission and the diff, so there shouldn't be a vibration issue there.
Now, how to mount it. Are you planning on having lots of passengers? It might not be that important if not. There are smaller seats, albeit less supportive, that'll fit in the smaller space, but at 14.5 inches, it might only be a technically called a seat. Another idea is to build a bench type setup for the passenger, using up all available inches, then leave the racey stuff for the driver.
I believe Keith T. had an interesting solution to the PPF mounting for the diff.
I guess this is where you can let the creative juices flow (big help, I know)