I would find out whether it's electrical or mechanical to narrow the search. First, have ONLY the battery cable on the starter and run two jumper wires---one from the battery cable to the + side of the coil (or to the "BAT" terminal on an HEI. Have the second jumper on the "S" terminal of the solenoid (one closest to the block) and touch it to the battery cable to start the engine. Start and then pull the jumper from the coil or HEI to kill. If it doesn't hang it's in the electrical system and if it does it's mechanical. You cannot move a bellhousing mount starter shallower or deeper as far as tooth mesh is concerned. The machined snout fits the hole in the bellhousing exactly so all that can be done is to shim the starter out from bellhousing. The bolts simply hold the starter to the bellhousing, the machined snout locates it.
