Flat top pistons in a zero deck application and 63 cc chambers will net you about 11.8:1 compression with a 4" arm. You have heads that are capable of supporting the inches so I'd go big. You might be able to run a lower (numerically) gear set to keep the rpms down and make things a little less peaky. I would upgrade the I beam rods to some H beams though. Unless they're something other than Eagle SIR or the better but still not quite up to par Scat 4340 I beam rods. I like the Scat I beams, but the bolts aren't as beefy as I'd like to see. If you want to spin the motor, upgrade to Program splayed caps or to a better block. Stock block will handle a lot, but they do have limits.