I guess you already have the Coronet, but an a-body would be an easier place to start and I'd really be thinking about an overdrive 833.

Now, what I have chosen for fuel mileage is k-car derivatives, specifically LeBaron convertibles. They are cheap and turbo 2.2 with manual transmissions are out there. They get about 20mpg in town and 30+ on the highway (it seems everywhere but here in California where they reformulated our fuel to get worse economy). The LaBarons are also big enough for reasonably tall people.