Here is my take on the two cars:
The RX7 is a beautiful engine, wrapped up in a really poorly built car. The construction materials are terrible, plastics are brittle and the control switches are flower-delicate. The transmissions are noisy and hot, and the electrical motors are obviously lowest-bidder-wins-the-contract. That said, the FC is a well balanced car that handles like a lotus.
The Z is a true sports car. While it was light in it's day, today it's a tank. I mean you can still drive it and flick it around, but its more "planted" like the Supra. The engine weighs as much as a fuckin battleship. The brakes are weak and under-worked. The HP is warm at best, and the balance is very ... I dunno.. oversteer heaven. When you hit the brakes you can feel the car rising from it's center of gravity, and getting very squirrelly on you. It's a hammer approach to chopping down a tree, whereas the RX7 is a proven Axe.
V8 swaps into the Z is common.. but .. why. Unless it's been your dream ... DREAM ... to drive a V8 powered Z, then you'll lose interest when you see all you have to do to make it fit. It's not just "get an adaptor kit". With a manufactured, measured and mass-produced kit, the swap costs alone, no engine or car, will run you to $3000. Or, you can fab everything yourself and have it take a year to complete. Either way, you'll leave the shop without a feeling of accomplishment and from what I've seen of you (the fact that you have told us about 40 times "I'm getting a new ________ car!") you won't finish the project.
Now the RX7, with a large turbo and proper tolerances / effort in design will yield fast results - results you can understand.
-> Steve
Note: I am biased to the RX7. I don't have a problem with, I just don't like Zs too much.