Timing suggestions for 360?

You are kind of stuck in that situation. The factory style distributors never used a vacuum pod that was designed to work on real low manifold vacuum levels.

You may want to look at one of the distributors that have a digitally controlled timing curve. I do recall seeing an artical on one that you connected the manifold vacuum source to it and you adjusted the vacuum advance with a poteniometer like the adjustment for total and rate.