When I put my new BDR together I started having the same problems that you are when I started adding electrical accessories. My batt was fine, all of my connections were fine but the voltage drop when everything was turned on was draining my batt. Just to clarify, I'm running 3 fans, fuel pump, crankcase vacuum, FI, MSD and of course the lights. My older powermaster just was not keeping up. Once I removed it I found black soot where it had been trying its best and was just about to fry. After a whole lot of internet snooping I went with one of these:
Chrome 170 amp S series alternator for 1963 - 1985 GM - Mechman Alternators
The difference is in the low end charging for me. These are 6 phase alternators rather than 3 phase and now I never have an issue. Even with everything on I’m getting 13.5 volts to the battery at my 1000 rpm idle.