Originally Posted by Trevorh
The more I think about it, I'm not convinced that the central bit of the firewall was doing much anyway so it's maybe not such a loss having to cut it out. The main worry is that the bolt holes will be very close to the big hole, so reinforcing the edge of the hole to stop the bolts tearing out sounds like a good idea. Of course, the T nut flanges will protrude into the hole so will need to be ground away to let the motor through.
Agreed the mount centre just adds a little stiffness - not critical, and torsional stresses are mainly applied in rapid acceleration, so my view is that someone gentle on the throttle such as yourself would not need to worry about it unduly.
How about front mount with installation from the rear, cut away only as much of the mount plate is required to ensure good containment for your T-nuts and save having to grind some of them off?
Actually I would use wood screws (again from the rear) anyway, but some clearance is still required so the ply doesn't fray at the edges.
I showed my 12-year old grandson some of your posted pictures yesterday - you should have seen his face! I think he was quite impressed.