Installed an electronic distributor on my slant and now I can get enough advance to set the timing. The bracket with the two slots is at maximum adjustment. Am I missing a blinding flash of the obvious?

When you say the bracket with the two slots is at maximum adjustment, does that mean you already loosened the bolt holding the bracket to the underside of the distributor body and moved it the correct(!) direction towards the direction you need to turn the distributor? If so, it means you've installed the distributor one tooth off from where it should be.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

I loosed the bolt on the body and tried to adjust so the slot for the hold down will allow me to give it more advance. Right now whne I hit it with the timing light the mark is at the bottom of the tab. Its abot 12 degrees BTDC I can't move the distributor any further (vac cannister moves toward block to advace

As was said, LIFT the distributor to disengage it from the drive sprocket. Rotate slightly in the direction you need to. Then try to set the timing by rotating the distributor.

I was able to get it fairly close. It's about 5 degrees BTDC. I'll try and adjust this weekend. Since I need to retard it slightly and the rotor spins clockwise my guess is that I have to move the rotor counterclockwise one tooth.