'77 K10 won't start

[79BIG10]

I wouldn't hurt to replace it with the one you have that is working and test the other one. Another thing that would hurt is that if the battery was hooked up wrong. Jody's dad bought a Bronco II for $250 since it wouldn't run etc. We hauled it home and after changing the stuff you already did and a few other minor issues (unrelated issues to the battery being installed backwards and trying to start) we had it going. We had the same problem though with about the same voltage you have to the HEI but nada after that. Also you might check the coil while looking into it.