Just for comparison, my 511 big block, .600 lift cam, 830 Holly DP gets 10 mpg over all. With 35 degrees of total timing with a very fast advance, all in at idle.

What I would do is re curve the dist for a faster advance, Set a total advance of around 35 degrees revved up.

This will give you around 15 degrees at idle. This is what the stock MP dist start with. If you don't have any pinging with this, then try adding the vacuum advance also for cruise speeds.