RE: False firewall
The fuselage is wide enough, so you might consider the very wide hardwood beam and mounting plate scheme used in the old Midwest TriSquare.
Basically, you fit some very wide hardwood beam mounts glued to ply nose doublers, then bolt down a "U" shaped aluminum or 1/4" plywood plate that the engine is attached to to the hardwood beams. The firewall needs to be notched to accommodate the beams.
The advantage of this scheme is there's no futzing inside the nose area when blind nuts come loose because all the bolting on an engine swap happens out in the clear in front of the firewall, the heavy strain on the firewall glue joints is distributed better to the fuselage without imposing a bending load on the center of the firewall like the stock setup does where the nylon/alum mounts are narrower than the firewall.
If you have access to a bandsaw, aluminum mount plates are pretty easy to fab. It can be done with a hacksaw, just takes longer.