If the timing is correct, and everything else seems up to par. Add grounds. Run the factory ground to the block, not the alternator bracket. Next run another ground from the block to the frame. You can use the brake line hold down under the engine on the cross over frame. Make sure you clean the contact areas real well. Check the factory grounds, there should be one on the firewill to the intake, make sure it`s connected and clean.
This is what I had to do to my Olds cutlass surpreme. I had a header pipe that rode on the starter so it got hot fast. After I added grounds no more starter problems. The added benefit was the lights were brighter and the alternator didn`t have to work as hard to provide the same power.