OK so I finally got the chance to tinker with the distributor today (BTW JimG, you were right it is a Mallory not MSD as I had been told). I decided that since it seemed to have 30 degrees of adjustment and I only wanted 20 that I should adjust it to about 2/3rds the distance of the slot.

What can I say, I got lucky and nailed it the first time.

Thanks for the sugestions everyone.