The 1st box melting down sounds more like a failed ground then a bad msd box. It very well could be the same issue with the second box. I had a msd 6a in my 82 and it was flawless.
I second bench testing it. If it test out ok then it would be a good idea to run the grounds to the frame instead of the body. Also where the 2 wires from the msd go to the wiring harness need to be solid.
Its 2 wires if your running a tach adapter.
