Vacuum/ in gear idle issue.

13* initial should whip over on the starter no problem. I'd check the pos/neg cables because the ends will often go bad. My Deisel PU did this. Acted like the battery was dead. The wires where they entered the ends were toast. Replaced all the ends and the truck wings over with ease.

Camshaft plays a big part in initial timing. My stock 340's get ~16 initial. Bigger camshafts into the 20's or locked out.

The vacuum gauge method mentioned is a good way to find your initial point as long as your electrical system is up too snuff. Each time you bump the initial, you need to reset the idle speed as it will likely pick up and observe the reading. As long as the starter will crank it over with additional initial timing, you're good. Then you get to fix the mechanical part of your distributor.