Miles per gallon?

As others have said. Check compression, and valve lash. Then check and make sure the vacuum advance on the distributer is working. Check to make sure there is no smell of fuel in the oil (leaking fuel pump), and PCV system is working.
PS: Make sure the timing marks are accurate. The dampers are know to slip after all these years.
I get upper 20's on my 64 valiant, but it was rebuilt for economy.