Clearly one cannot weld the steel to aluminum. "Staking"n teeth or tabs are all roughly the same idea.
I kind of hate the idea of having to access the motor through the back seat. Although if one wanted to do a sleeper, a 4-door would play the part nicely.
Ah well, some day.
Still thinking about this. What about a Honda H22/23a as a swap candidate? some versions made around 200 HP, which would be a nice upgrade from the Corvair motor along with the high revline and smoothness. No, KEP doesn't make an adapter.
With stock Corvair transaxle, you'll be glad to have that Honda engine if you drive it at today's typical freeway speeds. :-)