That's a tough one, isn't it? What about hardwood beams as a motor mount? I think maple was/is popular. Maybe something similar to the Pup arrangement but without that crazy box. You could support them on the backside and front of the firewall with triangular gussets and maybe another former an inch or so behind the original firewall. It would not need to be a full former so that would leave room for the fuel tank . If you extend the beams back far enough the tank could rest on top of them. Trying to accomodate that motor mount is going to be tough.

On second thought maybe that box from the Pup would be just the ticket....You could cut away parts of the firewall to clear the engine..once assembled I don't think strength would be an issue...

Randy