Slant 6 engine ignition timing stays retarded.

I'm tuning the slant in my 1973 Dart. I did all the usual stuff---- plugged the distributor vacuum hose after disconnecting. Set idle at 750 rpm. Adjusted mixture to highest vacuum @ 18" with a rock steady needle.
With the distributor rotated all the way counter-clockwise (confined by the slot), the best I can get is 5 degrees ATDC. It runs like a turd. With the vacuum advance reconnected it runs much better of course, but I can tell something isn't quite right. I've been racking my brains for days but can't solve this. I considered lenghtening the slot. I figured it's time to ask the guys who know way more than I do. Thank you.