/// Don's advise is on point. Dirty joints are more important than the size of the wire. There are tables of resistance per foot and hunderd feet for copper wire. One gauge will not make much difference. to double the capacity you must quadruple the crossection area of the wire. Over years the crimps on the end of the wires can corode and I suspect that the changing of wire size accidentally increased the output by cleaning up the end crimps (new plugs etc). Toward this end you might remove the Fast-on connectors and solder them to the wires. Even the two and three wire plugs can be dissassembled and soldered. Look at the plug and you will see a small tang that locks the Fast-on in the plastic or nylon plug. you can file a 50 cent screwdriver to make a tool that will depress that tang and allow dissassembley.

The other item, the alternator, can be tested almost any where. Standard battery checks normally include alternator output test.

Belts ---very important--- Energy is required in to get energy out. As the demand increases the alternator becomes very very hard to turn and if the belts are worn they will slip. Just tightening the belt may not help. But with the engine off (repeat off) go to the alternator and see if you can rotate the alternator shaft by pushing on one of the fan fins. If you can replace the belts -- both at the same time. Belts work by bulging the sides of the "V" as they are bent around the pulley. As the "V" wears down and there is no bulge the grip fails.