1975 Chevy Nova car won't start

1975 Chevy Nova 6 cyl Two Wheel Drive Automatic? Miles

I have a 1975 Chevy Nova with an inline 6 about a week ago I replaced the alternator seen that my battery died out of no where, and just now the battery died again and did not want to crank, I went back to the seller thinking they gave me a faulty alternater, they tested the alternater and it was fine they recharged the battery and put it back in and did not want to start up, I also have a 1976 Chevy Nova with a 305 V8, I have had problems starting it, it had to do something with the timing, I turned it on, put it on reverse and it died on me the battery drained, (new battery about 6months old) and went to test the alternator and it was good, also recharged the battery and it still wouldn't even crank just like my 1975, If you could please help I would appreciate it, thanks!