You are getting ready to spend allot of money on your engine. As such, if you are going to be spraying N20 in any quantity, I would spend the extra $100.00 and put a front cap on. That cap is pretty wimpy and tend to crack at the dowel holes. The shock load from N20 will aggravate that situation. No need to worry about #5. It is a big, beefy cap and has 9/16" fasteners. Piston wise, and there is some disagreement in the forums, I would use 2618 T6 alloy in any race application. The high silicon 4000 series alloys are fine for street cars. For further information about why 2618 is the preferred material for race pistons, look over the tech briefs in the
www.bmeltd.com web site. Good luck with your build.