FlyEng,

I don't have nearly the depth of experience on this as Matt, but the photo below shows about how much room there is to work with - not much!! None of our custom mounts are the same thickness, but I would imagine they fall within a 1/8" range. Depending upon which stand-offs you use (6.25" Syssa standard?) you could even end up with 1/4" less space than I did with my spliced stand-off arrangement which was right at 6" for the soft mount-to-thrust-washer distance.

In short, I think it will be a lot easier if you use Matt's method to tunnel in from the front.

BTW, After this photo was made I epoxied triangle stock into the corners where the motor box and firewall meet. It is very rigid and does not need cap pieces of ply for the top and bottom of the motor box. That sucker ain't goin' nowhere, especially with the soft mount taking out the sting.