The RX7 diff looks relatively small, at least in the 1st gen models. I'd guess smaller than 7", but it's been years since I've seen one out of the car. You might check the size of the ring gear online to verify my statment. Gearing is low as I recall, so that might make it less compatible with a V8 conversion.
As far as the weight is concerned, 2350 for a stock 240Z is tough to beat. For a race car, I'd go with the lightest weight car I could get, so that is going to be a 240.
The 3rd gen RX7 has some really good suspension and especially for a driver, I'd probably be inclined to go that way.
You should also consider rust. Won't be hard to find a rust free 3rd gen RX7. It will be much harder to find a rust free Z car, and fixing rust is time consuming and burns up $$$.