Check your engine sizes, I'm not aware of many Carb'd V8's that could fit comfortably in an FD body. Too shallow I think. That's why it's mostly done with FC's since they are more conventionally shaped under the hood.
Good luck though, the rotary is the keystone of RX7's but if it is better to have a V8 than no motor in one of those beautifiul babies.
One of the best parts of a V8 swap into an rx is the large amount of low end torque you'll get. Burnout city! So be prepared to slap a mean differential onto the car and don't just dive into a new engine.
oh also go here:
http://www.torquecentral.com/forumdisplay.php?forumid=3
lots of info on v8 swaps there