fo shitz and giggles, you may want to look into your coil, stator, and grounds too. even with the difference, it should be able to burn every bit of 300's above 115psi, with your mods. also, have the chambers measured in the head. if it was "milled" on a belt planer, rather than cut on a mill, the cut will be tapered tward the leading edge when placedd on the machine. this could cause the head to have different size chambers