Vacuum canister for road racing / AutoX is a bad idea!
It will be good for one, maybe two cycles of the brake before needing to "recharge". You will have a false sense of confidence until you have to negotiate a couple quick turns and run out of brakes. Not the same as street driving and stopping at a light with a big cam. Higher idle rpm and initial timing would help more than a canister.
For AutoX your carb will need a few mods to perform well. I tried to get a nice Proform 750 carb to work with road racing and AutoX for a year and finally just switched to fuel injection.