Chasing a gremlin

[Dead Parrot]

Check the battery cables for internal corrosion. The outside rubber can look great while the copper inside is rotten from years of acid exposure.

Check the junction block. If original, it is about 42 years old. Make sure everything is clean and not burnt.

2nd the check the grounds.

I don't know where the LS grounds the neg battery cable but the older SBC often used the alt bracket. If the bolts on that are loose and/or corroded, could explain loss of both battery and alt power at the same time.