distributor hold-down prevents timing adjustment?

Adjust the slot/bolt. Turn one tooth will be too much unless the actual problem is the distributor is installed one tooth off now.

To remove the distributor on a Slant motor, remove the distributor cap, rotate the engine (by bumping the starter or pulling the fan belt and fan) until the distributor rotor points somewhere memorable: straight up, straight forward, straight down, straight backward. Remove the coil-to-distributor wire, remove the hold-down bolt, and pull the distributor straight out. Watch how the rotor turns about 20 degrees counterclockwise.

Adjust the hold-up bolt (holds the plate up to the distributor), making sure to move it the right way. Always visualize: rotating the rightside-up distributor body counter-clockwise in relation to either bolt hole on the hold down plate will advance the timing. Clockwise will retard. Before you actually loosen the hold-up bolt, hold the distributor upright and correctly visualise which direction you need the hold-down slot to move. Take your time and don't get crossed up when you turn the distributor upside down to do the actual adjustment.

Take the opportunity to have a look at the distributor drive pinion while it's out. If it's cracked or broken or otherwise alarming (chewed, falling off, etc), time to replace it.

There are supposed to be two O-rings on the Slant distributor. One skinny one in a groove in the distributor shank, and one fatter one right where the dist body meets the block. Missing either of these = oil leaks.

To reinstall the distributor, line it up with the block hole, spin the rotor right back to where it was in step one before you loosened the hold-down bolt, then rotate it about 20 degrees counterclockwise from there, and push the distributor home. When the distributor seats on the block, the rotor should be pointing the direction you originally picked in step one. Install the hold-down bolt loosely, attach the coil wire, install the cap, start the engine, adjust the timing correctly (distributor vacuum advance hose disconnected and plugged at the distributor end, engine idling about 700 rpm). Tighten the hold-down, unplug and reattach the vacuum advance hose.