I think I have decided to use a Continental IO-550 engine. The engine mount bolts for the TCM engines come through the firewall in the lower corners, and the standard header tank interferes with that location. So I have to modify my beautiful header tank by cutting the corners off.
The cut line was marked, and the oscillating tool made short work of the cuts. Two thick lay ups (8x bid) were used to make the new corner walls. These were coated, sanded, and coated with EZ-poxy to form a fuel-tight inside wall, then were bonded to the header tank with a fillet of cabo and 2x bid around the perimeter. New mounting ears were also added.