Well, my 1975 D100, with a 360/727 combo, will not crank when the ignition switch is turned. It is making all the right connections in the system, even to the starter relay. That relay has also been replaced, but to no avail to get it to crank.

The starter only cranks when the two big terminals on the relay are jumped. What should I look at next? Replace the starter?

 

if that's the case the solenoid (the thing youre jumping) is likely bad. a trip to autozone and a 20 dollar bill should solve that.

 

Check the Neutral safety switch