20mpg without an overdrive is mostly in the 114LCA nitrous cam combined with a low tension ring package. Also low compression to allow spraying some HP on pump gas, the engine only cranks about 90psi on the starter. Should also work w/ a mopar.
Here's a couple videos of before/after installing a 'tamer, that part works with a mopar as well. Their goal is also 10's, they picked up .37 in the 60'...
Before-
After-
