Do the multi-meter tests first. You may have something draining your battery.

If your altenator is made as a one wire unit, it may be the problem. If it's a 3 wire unit and you're only running one wire, that's the problem.

The large red wire from the altenator is the battery charging wire.
One of the terminals goes to the gauge.
The other terminal is the sensor wire. It runs to the fuse block and reads the voltage at the fuse block where all the electrical items feed from. If this voltage reads less than 12.6 then it will trigger the altenator to charge the battery to the proper voltage, so that all the electrical items can get a 12.6 voltage to work properly. Some one wire altenators have an internal wire from this terminal to the large red wire terminal. As the battery drains, it constantly reads anywhere from 12.6 to 18 volts, and won't trigger until the altenator reaches high speed.

Scenario:
3 wire altenator,
Proper voltage and charging rate at the battery. Proper voltage at all times at the fuse block. Lights nice and bright, 12.6 volts at the HEI distributor giving a good 20,000 volts to the spark plugs and a good clean burn.

1 wire altenator,
Improper voltage and charging rate at the battery. Low voltage at the fuse block. Lights dim, radio low, horn sounds like it's dying. Only 9.6 volts at the HEI distributor giving a mediocre 15,000 volts to the spark plugs, resulting in an unclean burn, leaving behind deposits to cause detonation or pre-ignition, poor excelleration, and poor MPG's

Like I said, do the multi-meter tests first, then if you still have the original wiring, swap out the one wire altenator for a 3 wire altenator and see if that doesn't clear up the problems.