The Only 2 Radiator Manufacturers that I have used on my FD RX-7s are KOYO and MISHIMOTO.
The main reason being that when we start customizing our cars - They become Project or Kit Cars.
And from my experience - The biggest problems I've had with engine swaps are Engines OverHeating.
Since your car is the First Gen RX-7 Mishimoto does not make a Radiator for the First Gen RX-7.
In My Opinion - Your Best Option here is the KOYO Radiator.
Here's a Picture - And depending what your budget is - You can make the best decision.
You can get this one from RAMEN Tuning.
1225 Kingsway Road
Brandon, Florida 33510
813-822-3578
https://www.ramentuning.com/You can purchase this OnLine.
But I suggest you call them and let them know you're a first time Customer and they can give you a discount.
I hope my Input was helpful.