Rot, I am building a GP stuka also but putting a ST .90 in it. But last year I brought my Platt Focke Wulf 190D-9 out of retirment and Had the same problems you are. What I ended up doing was buying Sig 60 size motor mounts (made of aluminum but with no cross member behind the engine. Then I had to dremel a hole in the pressent firewall to allow the engine to move back to a reasonable prop position. Once this was all done I made a box to cover the hole I made in the firewall.

I made the box by cutting a oversize peice of 1/8 marine ply (Bass) that fit over the hole very nicely. I used 1/8X3/4 Balsa stock to space the secondary firewall from the original, I ran the grain parallel to the firewalls (because it did not require any sanding that way). Once assembled I covered all the balsa on the engine side of the box with 1/64 marine play for strength.

Good luck!
Joe