distributor hold-down prevents timing adjustment?

Timing at 0 deg BTDC sounds a bit weak. Might have been for emissions in a later slant. I would start w/ 5 deg BTDC. If idling lean, it would like even more advance. The main disadvantage of too much initial timing is that when it fires before TDC, it fights the starter when cranking. With computer spark control (engines since ~1995), it can fire at TDC (or later) when cranking, then switch to what works good at 600 rpm idle.

Note that the gear teeth are slanted, so as you lower the distributor the rotor turns a bit, so allow windage for that. I never had to adjust the lower bolt on a slant distributor. Maybe I should have, but I always got one into adjustment range by rotating the rotor one tooth. I don't think I even knew the bottom bolt had an adjustment slot back then.