I'm not saying it is for sure a leaking manifold, though anything's possible. I actually came back to this thread on account of I thought of another thing that could make your mileage drop, and I see that you have detailed a related issue in this post, namely your exhaust.
I had a 72 Chevy half ton that I bought in San Diego some years back, and the simple act of replacing everything in the exhaust system from the headers back made an AMAZING difference in the old 350 cid, and it didn't run all that bad previous. Exhaust leaks, collapsed inner exhaust pipes (the reason i was back on this thread), exhaust heat stoves like you describe stuck shut, can all contribute to poor mileage, and/or driveability problems. It's strange how well we think our old beasts run until we make some improvement, and have it run hugely better. Of course, as I talked about, sometimes changing one thing that was covering up something else could make it run worse until you work all or most of the issues out, so just be prepared for that.
What part of So. CA are you in?
I wouldn't get too worked up about the distributor just yet...it sounds like you have more immediate issues to take care of first...though, as I said, in combination, it could be contributing to the overall mpg problem.
