by far the easiest is to add to the header. start there adding like 5 or six inches, then try adding to the belly an inch or two at a time. with those cone angles being equal, there wont be a huge hit, but there will be a very obvious power gains. though not necessarily on the first try. the variable here is your exhaust port timing, but remember that the longer the distance that the pressure wave has to travel, the lower rpm the "power band" will be. with a 32 inch pipe, your peak power is somewhere around 13-14000 rpms.
@slowbalt - my bike also does 39mph with a 41t sprocket and 24.5in tire. which puts me at 9500
