Jeez you had a horrible day! That truck look super clean for a 78. I am well familiar with those things. The factory amp meter in the dash needs to be bypassed if it hasn't already been done- take loose one of the wires on back (2 terminals) and bolt the two wires together on the same terminal. Thats the simple way around it. They tend to melt down and sometimes try to melt the truck down too. Also, instead of having an elaborate fuse block system, they have "Fusible Links" which are sections of special wire that burn out at certain amounts of amperage passing through them. Really piss poor engineering, but thats what everyone was doing back in the 70's. They just hadn't figured out a better way yet. Anyway there's several of those links they are all unmarked and pretty much no way to know which one is causing the problem you just have to get a test light out and start probing around to see which one isn't passing power through it.
If you remove the alternator they will test it for free at Advance Auto Parts, other parts stores usually offer that too but Advance is my favorite place to go from my options here. If it tests out good, you can either A) throw a new voltage regulator at it to see if that is the problem , or B) test all the wiring for the regulator and see if that's the problem, if its all good then the regulator is probably at fault. (note that there are two different regulators, one is two pin connector, the other is three pin connector, both connectors look alike). I am pretty sure the power from the charging system goes through a fusible link before it hits the battery so definitely check that out really good. Hopefully your mechanic is somewhat familiar with fusible links, as I said they are very ancient technology and some guys that don't normally work on 80's or older stuff probably have never heard of or seen them before. When fusible links blow, its normally a sign of a bad ground or a bad voltage regulator. A bad ground would cause it to draw more amperage through the "hot" wire (in turn travelling through the fusible link) and a bad regulator can let the system overcharge, causing too much amperage to travel through the links.
Hopefully this helps! Oh, also, cruise over to ramchargercentral.com you may find something there, they have a good diagnostic/troubleshooting section that might help and lots of helpful members. I'm "BushHogBoy" on there
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