The braking system on any vehicle must be considered as a whole, not as individual parts, to determine your needs. Start at the brake pedals,determine how much force you are putting to the master cylinder (pedal ratio,most systems have about 6-1), then the size (bore)of the master cylinders and the displacement needed to operate the calliper pistons. Consider whether the force required to operate the brakes needs additional assistance, most well designed systems should not need any! Secondly, if you are using multiple carbs or individual cylinder intakes (such as webers etc.)it is unlikely that you will get sufficient vacuum to operate two servos(boosters). Dont just fit boosters and hope its right,work it out first, its not that difficult.