With that kind of power turning some big rubber, you definitely need the heavy duty stuff. Brian is an engineer and you can take his advice as gospel: If you are using a single point connection (ladder bars or "truck arms"), go with the 3/4"; if you are building an adjustable four-link like a pro-stock car, go with the 5/8". Don't use the economy stuff either; go for the pro series (Speedway Motors) or better.

Heim ends have matching shank and hole sizes. For example, the 5/8" rod ends have a 5/8" hole and a 5/8" fine thread shank. The thickness may vary between different series ends and different manufacturers. Before you fabricate any bracketry, you should get the rod ends you are going to use and build accordingly.