I'd start with the tried & true "vac gauge" method: keep advancing the timing till you reach max vac all the while reducing the idle speed back to the lowest point it will idle in gear at then reduce the timing till the vac drops (1) in hg & that is the ideal initial timing if it does not kick back when starting. If so you can either add an on/off ign switch & get it cranking with the regular ign switch then switch on the auxillary ign sw so it gets power & will fire or reduce the initial 1 degree & retry and another 1 degree if needed