Re: New to the forum
Run your piston up to TDC with the intake manifold off and then color in what's left showing on the piston with a sharpie pen. Then pull the jug and slug and trim off the marker plus a little bit and smooth it all out. You also might get away with drilling the wrist pin bosses a bit to lighten the piston. You got the GT5 so you already have the hi compression head and domed piston. I wouldn't recommend polishing the dome of the head as that will diminish the turbulence that promotes atomization and good combustion.
Another thing I've noticed is that the expansion chambers from all but Pypeline are designed for 50cc motors. If I were you, I'd add 2-3 inches of tubing in the center section of the pipe to "tune" it to a length more suitable for a 66cc motor.
Make sure you have hose clamps on the tubing of the boost bottle and that all connections are sealed. Also a smear of RVT inside the intake before you slide in the carcb will alleviate vacuum leaks in that area.