As far as pistons go, you'll need to look for ones that will give you around 10:1 compression. The Keith Black hypereutectic pistons (KB237) aren't too pricey and will probably get you around 10.75:1 with the 915s, which is a little high, but if you're using the right cam (and maybe a thick head gasket), you'll probably be OK on pump gas.
You may want to repost this in the Unlawfl Race section. They can give some good advice on cam selection, etc.
Good luck.