eBay score!
My engine came with an SAE #5 bellhousing. The bellhousing adds about 5 inches of length to the engine so I knew I would not be able to use it.
I scored this Kubota backing plate on eBay for a few dollars. I don't think I'll be able to use it in the car, but with a set of transfer punches I'll be able to get the critical holes marked out on the new aluminum adapter. (I've decided I can build the adapter my self. this ought to be good
)
For the actual in car adapter, I found a 13"x13" plate of 1/2" aluminum for 70 dollars. A bit steep, but a possibility. I'll keep searching.