X heads on a 360?

They should be around the same cc. Shouldn't effect cr, the extra air flow will help power wise, I wouldn't go to large of cam cause your engine is a low cr engine.
The more gear and stall will allow more cam. Probably something like xe262h to xe268h cam or similar depending on rest of combo.

I'm running a summit brand cam duration 285/285, Lift .480/.480,