Yeah, that's all I know of for the s chassis. It's a fair upgrade over the stock one on the cheap. Ive seen some dB competition cars back in my youth; battery banks, lexan windows, and dynomat for days. The Blazer I'm thinking of had to plug in an external charger to top it all off but he was running 8 subs I think and several amp. For just driving around and listening, i think in these cars at least, you could get PLENTY of good sound from a quality setup with a single 10 like a JL W7 in an enclosed box and quality 500w amp. Those 15s even in an isometric setup are going to need a huge box and don't generally need/want an amp of the wattage OP is asking about.
I realize that's not his question and he's going to do what he's going to do. His question may be better answered on a dedicated mobile audio site where someone knows of a place that rebuilds alternators for higher amperage. All I've got in my repertoire is that Quest upgrade.