61 Dodge Lancer 6 to ? conversion

The car has a slant 6 and they want Better fuel mileage?

Hmm, buy a Prius.

older Lancer/ , and pre 1963 Valiant;
you can have a V8, but not a heater.

Find another 225;

Electronic ignition, and a full tuneup,
including adjusting the rockers, might help.

do a compression test, After adjusting the valves.