If the belt were slipping I’d imagine you would notice other components slipping first. Think about it, by having one under sized pulley potentially loosens the whole system. And sure a smaller pulley decreases the wrap on the alternator by a very small amount, but tons of people have made this change without issues. The belt tensioner is normally more the capable of handling one pulley change. I certainly hope you checked out the belt and tensioner condition as the HO alternator was installed.
I would probably start by checking all your big electrical connections, and break out the old multi-meter checking for voltage drops. The fender well ground on all GPs are a POS, it is always a good place to start. Next I’d be looking at a nice battery. I’d aim for about 800CCA. And of course I have made the assumption you are running proper wire sizes to support the audio equipment.
Good hunting.
