I personally dont think new cars, nor electric is the way to go, cars from the late 80 early 90 were much much lighter, and still "Safe" they could be bought cheap, driven hypermiled in order be driven cheap, etc. Even electric versions of these cars with newer technology would be better than converting a heavier new car.

ideally, my friend built a vw beetle from the 60s. he converted the engine to run on two cylinder, and got 60-70 mpg. He didnt even go as "light" as he could as he converted using upsized jugs. He changed the gearing in the transmission to keep it quick enough for highway use, as well as get to keep speed on the highway (slow up hills though). And it all cost him less the $3000.