You might have corrosion on the wiring connectors in the wiring bulkhead (where the wires pass through the firewall). The big red power wire that passes through the bulkhead is notorious for giving problems on E-bodies and 71+ B-bodies. One problem with the big red power wire is, often someone will replace the alternator with a higher output unit, which overloads the wire at the bulkhead, which leads to hair loss problems down the road....
I have had problems in the past with burning up the alternator field wires too. I suspect that is the primary problem in your case. The only way to fix them is to trace them and replace them, if one or both is showing excessive resistance.
Also, the voltage regulator is a regular PITA when you have a high output ignition. To solve all problems (regulator, field wires, and power wire), I bought a PowerMaster one-wire high output alternator and ran the alternator single wire directly to the battery. Now, I have lots of juice, all the time.
One other thing to check is the ground wires. You need a good ground from the battery to the frame, and from the block to the frame. Make sure you have clean contact with bare metal on each end of the ground cables, and use fatty cable.
Hope this helps.
