First, you have to decide what the safe rpm limit for your motor is. How hard can you spin it with out breaking it?
The maximum allowable rpm for Imca is 7000 rpm. Limiting the rpm to less than 7000 is probably going to limit the potential power output of the engine. That said whatever you decide is safe is what you need to tell the cam manufacturer when you call them for a recomendation. Also you need to tell them the intended use. How big the tracks are that you will race on. What your car weighs. Your experience level. Etc. Check with a few different manufacturers and compare their recommendations then go with whoever you feel comfortable with. Be honest with them, if your only going to turn 6000 rpm you don't want a cam recommendation for a motor that turns 7000 rpm. Good luck!
[Edit] P.S. I suggest you read over the engine section of the rules before you call them so you can tell them what you can and cannot do.