I'd say Oger is on the right track, place voltmeter across battery terminals, observe cranking voltage while cranking for max 15 seconds.
If less than than 10 volts, get a bigger battery. If not less than 10 volts, replace cables with welding cable, 2/0 AWG. Also, right after cranking, feel with hand all connections for heat. When you feel heat, that is the bad connection, either dirty, or too small in size( not enough contact area).
Another idea, engine cold, take beam torque wrench and measure how much to turn other and compare to heated condition, could tell alot right there. Set initial timing for 6-8 deg BTDC.