The very first thing you want to do is very carefully check that all the battery cable connections are clean and tight.
Years ago, I kept replacing parts: battery, then starter (not fun even in a carb car) even cables. Finally had to put in a second ground cable from the battery to the engine block.
Last week, I finally solved the erratic starting problem I have been having in my Dodge Van by cleaning and tightening both ends of the battery ground cable, even though all looked clean and was tight to start with.