distributor hold-down prevents timing adjustment?

ok, the slant has been running a little rough, so i decided to check the timing. the underhood info says that it should be at 0*, which i think is odd, but whatever. when i checked, the mark on the damper was to the RIGHT of the mark on the block. when i loosened the hold-down bolt, i found that the distributor would only rotate clockwise, bringing the mark further to the RIGHT.

there's a slot in the hold-down that only lets the distributor advance so far. i want to advance the timing to the point where the two marks line up, right? when i blipped the throttle, the timing mark jumped to the LEFT of the block mark. she runs smooth at higher rpms, but kinda stumbly at idle. idle rpm IS a little low, 650.

do i have to re-clock my distributor, or is there an easier way to do this? i feel like i'm missing something here.
and yes, the pickup was on the #1 cylinder. (front of engine)