car won't crank.

I can probably figure this out given time, but if some of you guys can help me out I would appreciate it.
I have a 65 Valiant with a 68 318 engine with the original automatic transmission in it that was rebuilt at some point before I got the car. The car will crank if I take the spark plugs out, but once I put the plugs back in it only clicks. I assumed that the started was worn out. So, I took the starter out and put a very good used starter in that tested well and... same issue. I've cleaned the connections that I can see on the firewall but no difference. What should I look for next?