So anyway, if you are going to use the plate, you can still just use studs to mount it. Make up studs to fit the engine, and size them to be long enough to go into the block, with enough sticking out to go through a small flat or lock washer, a nut, another flat washer, your mounting plate, and then through washers and nuts to hold the plate on. That will hold the engine backplate on, while still giving you a surface and means to mount your mounting plate to... You say you are going to use spacers, so there should be room for the washers and nuts that will hold the plate to the engine.

AV8TOR