Ok, this is where you are going to make power, modding the cyl head, that & a decent camshaft.
Larger valves, lumps pocket/bowl porting minimum. It is going to cost though. It is not cheap period.
If you are going w/307 flat top pistons (w/no chamfer on the top sides) you need a better cam to complement all the others items you are going or intend to add.
It all depends on what you are looking to get out of it.
MBHD