Upgrading Alternator

You are lucky that on your car, the alternator current is routed through solid lugs instead of thru the connector blades ("56 series terminals") of later models. The later have a tendancy to melt the bulkhead connector. If you disconnected the wires, sanded connections and re-attached w/ silicone grease coating, it should be fine.

You shouldn't get a 2.8 V drop from the battery to the ACC fuse from just the wiper current. Besides the bulkhead lug, that flows thru the ignition switch to the ACC terminal, which should be a 12 awg black wire at the switch. Remove your switch and sand those terminals. The battery feed is a 12 awg red wire. If that doesn't help your switch may be bad inside. Above is based on the wiring for my 65 Dart.

The 0.7 V drop thru the ACC fuse sounds a bit high. Remove and sand the connections. Perhaps the wiper motor is drawing too much current. A stalled motor can draw a lot of current, making it over-heat and melt windings. Try running ~14 awg jumpers straight from the battery to the wiper motor and see if it jitters w/ a full 12 V supplied. If so, the motor is probably bad or too much friction in the linkage and pivots.

As others said, the alternator is not the problem. You should be able to run the wipers fine just off the battery. Your battery is barely drawn down by the wiper current. A fully charged battery should read ~13.4 V, so either it isn't charging fully or your multi-meter is off. I have the factory 30A alternator in my Newport and it keeps up fine w/ electronic ignition, TBI fuel, and headlights on dark winter days, and I only drive 4 miles between starts plus many 1 mile trips around work. The later square-back alternator puts out more current and is direct mechanical fit, but you need to ground one field terminal or switch to the electronic regulator (better). Some install a GM-type "1-wire" alternator, which cleans up your wiring, but requires mounting spacers and a special tensioner arm.