It seems that SBCs in RX7s are so common that there is an entire community devoted to them with a coming book and DVD!
http://www.v8rx7forum.com/
This means I'd either
a) rather do something else
b) it's gott be a little quirky, like an E85 TT V8
Continue discussion. I'll make a decision once I see all the major issues are on the table and can see a path to the best option.