I've lost count of how many times I've taken my distributor out. I have a unilite and a spring kit and had to experiment quite a bit.
I've had the best luck with 34-36 total timing; your 28 may not be enough.
I found these to be a good read:
Distributor Advance Explained
Timing 101

The issue i had at cruising speed [low rpm] is the ported vac increased and I'd get a ping. I ended up running without the vac can; unfortunately Mallory doesn't make an adjustable can for the unilite. After reading the article above I'll try manifold vacuum...