you should always bypass the ammeter gauge esecially when you go to a higher output alternator.
the old gauges give the problems not just the wiring.
to figure what amp alter. you need, try to get the power usage of the msd, the fuel pump, fans, etc. and add that to your basic stock rating of 60 amps . i would think that 120 to 140 would be enough. run a large gauge wire #6 or #4 to your battery from the alternator also .