Cam selection help please

Your problem isn't with the piston to valve clearance, it's your static compression ratio. If you go to a longer duration cam you will bleed off too much compression and then you actually make less power.

Every cam has a suggested static compression ratio. If you go above it your motor will ping bad, if you go below it it won't make any power.

If you want max performance without having to change compression ratio (pistons), call someone like Clay Smith Cams and explain what you have and what you're trying to do and they can tell you your best option. I'm guessing it's gonna be to match a cam with your engine to get the most out of it and then go to a different rear gear to take advantage of your engines power band. Right now you have no high speed ability.