With those kinda specs in a 499 I'd expect to be able to get more than enough vacuum to run the brakes... What is your base timing set at? More intial timing will strenghten the vacuum & improve the idle & low speed performance.. But to run more intial you need to limit the total so you don't wind up with 40+ degrees of total timing when the mechanical advance is all in... Typically I try adding more intial without worrying about total at first till I see how much intial the motor wants then I modify the advance slots to limit total to around 34-36..