I wonder how much of a gain I will get!

If the entire assembly was siezed, I would imagine that you will see a huge gain. Is it possible that the previous owner had the vacuum and mechanical advance locked out and was going with all of the advance 100% of the time?
I see your from Maine. Are you going to the Atlantic Nationals in Moncton, N.B. this weekend? If you are, a few of us will be there on Saturday and some on Sunday.

Jack