In short, it will be really difficult get 30mpg from stock-looking 40 year old cars unless you go the Japanese route with a first gen Celica, Plymouth Arrow, Honda Prelude etc. Those might get you 30mpg without engine swaps and will be really fun to drive. With engine swaps and suspension work they'll haul. Also look at first gen VW Sciroccos. That or a Rabbit with a, say, 1.8 turbo swap will get you where you want to go.
'84 Prelude was rated for 29mpg highway with the 1.8 5spd.
http://www.fueleconomy.gov/feg/Power...de&srchtyp=ymm
84 Dodge Charger 1.6 4 spd was rated for 35 hwy!
http://www.fueleconomy.gov/feg/Power...er&srchtyp=ymm
If you want a classic car, get a classic car. If you want good MPG get something with EFI. The only way you'll get both is fuel injection.