Check the battery cables too, especially the grounds. There should be a ground from the battery to the fender, body to frame by the pass. side motor mount, frame to block, and head to firewall. I like to replace the battery ground with a double wire cable, the thick cable I run to the block and with the smaller one, I attach it to the fender. The positive cable should be checked also, it carries the main current to the starter and the smaller wires power the accessories. Another problem that shows up is with the ignition switch itself, it carries all the current to the starter and the contacts can become burned. I added a relay to my starting circuit which only used the ign. switch power to turn the relay on and the relay would then supply battery power directly to the starter. No more problems at all after that.
Good luck
Jeff S.