I would talk to Tim Goolsby at Bullet cams, you may be near the point of port saturation (where you're using up the entire flow potential of the heads) but it's worth a shot to talk to him. Bullet owns the rights to the entire line of the old Ultradyne cams which are about the most violent power-making RACE cams I've ever worked with in my 32 years of mopars. Switching from a flat tappet grind to a solid roller cam should still be worth some power and your lobe spread may or may not be optimum for your particular combination. Generally a lighter deep geared 4 speeed car (which has less rpm drop between gears) can go to a wider seperation which lower mid range torque but can raise the RPM that peak torque and horsepower are achieved.

The flow potential of either head you have mentioned is going to be around 300 cfm (+/- 10) under all but the most skilled porting hands, this suggests just over 600-620 horsepower of flow potential so you may go faster with a bit more head and cam provided you have the induction and exhust to carry it.

Another great resource would be our own Dwayne Porter who goes by the name fast68plymouth here on Moparts, he's exceptionally cam-saavy as well

The 'small' 3.91" bore of the 318 often gets blamed for reducing performance compared to the much larger 340 4.04" bore, while a larger bore is advantageous in a wedge style head, the chevy 5.7" LS1 has the very same bore size and many near stock bore LS motors run very quick.