Thanks for the responses and suggestions. I think fabricating a standoff from wood is the ticket. I've managed to upload a couple of photos of the firewall (only took 45 minutes of moving files here and there) of the 15-25 size AT-6 on which I hope to mount the engine. The backplate fits within the outer ring on the frewall.

Thanks again for the replies, but I'm sure to have more questions to follow.