The gap at the top of the piston is usually wider to accomodate for piston expansion due to heat. Usually you take a measurement on the skirt just below the oil ring (for piston to wall clearance).
A super charger will push the fual back the line to the tank. Use Mad Darts' advice for pump and regulator selection. You can run on one and boost refence the other(s) to toggle on when boost starts. The non-running pumps will act as a deadhead and not allow fuel to pass by them.
Run the 3" as far as you can. In my testing, switching from a 2.5" downpipe (whole exhaust) and substituting in an 18" length of 3" down pipe and mergingit to 2.5"...that was good for 10RWHP.
No back pressure after the turbo is ideal. It helps spool up (faster). I run 3.5" to the axel and a 3"...