Tom,
I've read Jerry's posts and he seems to really know what he's talking about. I'm chiming in because I have a '66 Chevy shortbed stepside truck with a 292 and overdrive. My overdrive is a 4 speed manual shift. Like you I have 3.73 gears and I have 235/70R15s. Overdrive ratio is .73 so I turn about 2600 rpms at 80 mph, about 2200 at 70.
I get nowhere near 20 mpg. But I have a cam that's bigger than stock, headers and a 500 cfm 4 barrel. Problem with me is that it's really hard to keep it at only 70 with the torque of the 292.
You might as well throw you on a nice sounding exhaust while you're at it - I have a Flowmaster muffler with dual outlets. Sounds near-v8ish, really deep at idle and normal acceleration.
I think 20 mpg is feasible for you, with the right setup. Jerry was right about lugging. Mileage will really suck if you lug or gun it. The right sized carb makes all the difference in the world too. I probably get about 15 mpg when I run 70 and use ideal acceleration techniques.
-ftyler
