first thing ya need to do, is pick up a copy of Tex Smith's "The Complete Chrysler Hemi Engine Manual"" by Ron Ceridono.
it's been out of print for a while, but i think you may be able to pick up a copy on the bag. maybe elsewhere ? i haven't looked.

don't know what to tell ya on mpg. either adapt an A518 or use a gearvendor. stock engines in the cars they came with were not known for gas misers, but weren't too bad for the times.
put in a lighter vehicle, it would definitely help, but by how much, who knows ?

as to the unleaded gas issue, hardened exhaust valve seats would be a good idea, however, unless you are beating on it at the drags, or stop light to stop light, this may not be entirely necessary. chrysler used way better material in their engines than did the others, and you may be able to skip this step unless the seats are beat in bad, or the valves are burnt up.

and don't forget this : "hemis = $$$". they are not an inexpensive engine to build.