have a 69' pro street Camaro with 468ci bbc was running 2" primary headers into 3" collectors  using 2 chamber flowmasters. the system was 3" and ran mid way through car,      how much difference would a bigger engine say 572 to 632 and 4" about the same length as the old system.....of course the new engine will have to have bigger headers,  would somthing like the borla xr-1 series be out of the question?  car will see limited street use and would like to drive to local track to race.  

I understand the rule of thumb is 2.2 cfm of muffler flow per horsepower. Any less than that and you don't realize the full potential of the headers. Engine builders are using high capacity mufflers as an extention of the header collector and getting the most out of a tuned exhaust system. The overall length is something you'll have to tune at the track or on the dyno.
I haven't checked the specs on the Borla xr-1 (I have investigated several others) and with a 600+ CI engine making 750 hp(?) you'll require 800cfm+ flow per side! That's for an optimally spec'd system. Any less and you'll hurt performance. Although at 600 cubes, I don't know how you would tell!