how many watt stereo with 60 amp alternator?

60 amps= 780w at 13V. P(power in watts)=I(current)xV(voltage).

The thing to point out here is that's 60 amps at roughly 1800rpm or higher engine speed. At idle, it will be a lot lower, probably more like 20-30A. And you have to run the ignition, headlights, wipers, blower fan, etc. These alternators are just barely adequate for the stock setup.

My opinion would be to go to a 94A delco 12SI or maybe something even more modern, and of course you have to upgrade the wiring as well.