[jeffspower]

I'd read the spark plugs to determine if the carb is rich or lean. Either will cause poor MPG. Edelbrock carbs aren't a very good MPG carb, & are usually jetted a little rich so they will run fair out of the box on most stock to mild engines of any brand. Every engine needs jetted & calibrated differently. It could have too stiff of power valve springs, causing a rich mixture at low engine load. Specs on the springs & a vaccum gauge to determine your manifold vac. running down the road is where to start. Then play with jetting to get your plugs to show a very light brown around the edge of the porcelin. Or, better yet, put in a oxygen sensor & you can read what it's doing going down the road with a digital voltmeter.

If all checks out there, look at your timing curve. See what advance reading you get at cruising speed.

Better MPG might not take much money, just a bunch of time tuning. Do you still have the OE Q-jet? If it's in good shape- might be the best way to go. A decent tuned Q-jet will usually do better MPG than the best tuned Edelbrock.