seeing as it a tall deck i vote go 4.5" for the fact that you can run a 6.7 rod and get still a decent rod to stroke ratio.off the shelf pistons will be another benefit.with a 4.6" bore and 4.5" stroke your getting right up to 598 cubes!!!plus those are big heads so either go big cubes or wind up tighter a smaller cube motor.what are your rpm limits?compresssion?cam selection?im in the same boat i got tube chassis chevelle with a 572 (pump gas 10:1) and wanna run low 9's and spray deep in the 8's.with an over cammed mph of 143 on a 90 degree day first time at track ill be there no prob when i regrind cam to correct duration....if you build big cubes,you wont have to wind up real high!valve train HATES RPM!!!!