Are you running an electric cooling fan??, electric fuel pump?? Total output (Amps) is only part of the issue. As with an electric cooling fan what the Alt output is at idle is also an important consideration. An elec cooling fan is doing most of it's work at low engine speeds. The late model CSI type GM alt has very good output at idle and isn't that large or heavy. IMO don't skimp on your alt. If your alt has marginal output at low speeds (AMPS) then the voltage will suffer and drastically effect the performance and life expectancy of these elec. componants. If your building your own brackets do the math on your drive ratio. The Alt normally spins faster than the engine by 3 times. Check the max operating speed of the alt with the manufacture and again do the math at engine redline and see where you are at. If you spin the alt slow at idle it will drastically effect the out put. Check with the guys at Power Master
http://www.powermastermotorsports.com/tech_help.html