Big 3 or 5, more battery(ies) is the answer. Amp draw can only be made constant, so you don't have voltage variations, if you have it available full time. An HO alternator only gives you more amps at higher RPM'*. At an idle, or cruising it doesn't make a big enough difference for the equipment you are powering. Although any of what is suggested can make a difference, to get the max benefit and best performance takes enough amperage through all scenarios takes storage capacity.

smaller alt pulley for more amps at lower RPMs?

and paint is generally non-conductive...

GM rely'* on the threaded hole to be enough ground, it isn't when you starrt powering things like a 2000 watt amp, or my dual Viair compressors that powered my air ride. Under pulleying the alternator tends to burn them out as they spin faster than designed for. You have the HO altenator, Put an extra quality high amp battery in that has only connections to the alternator and your sound system and be done with it.

Yeah you're going to have to sand down the surface you grounded to get a better ground.

The alternator im planning on getting does 200 amps at 1500 rpm and 55-60% of that at idle.