My son and I had nearly twin cars. I had a '66 Coronet 500 with a 361, Mopar Performance ignition, factory 4-bbl intake, Edelbrock carb, RV cam, stock exhaust manifolds with dual pipes through stock mufflers. My son had a '66 Satellite with the exact same setup, and both cars had 3.23 gears on the same stock-size radial tires. The only engine differences were my heads had a mild port and polish and a .040 overbore, while his was stock.
Both cars went to the track quite often. His times were high 14s to low 15s. My times were mid to high 15s. On the highway, his Poly 318 got 18-20 mpg and my 361 averaged 14-15 mpg.
The stock 361 heads did not flow well with the small valves, even with the porting, and the exhaust manifolds were very restrictive. A set of stock 452 heads and '68 or later exhaust manifolds would have probably knocked off more than a second ET, and obviously, a better cam and headers with good flowing mufflers would have done even more. However, I doubt the gas mileage would improve much. The Poly design is simply more efficient.
This was thirty years ago, so it would be much easier to find performance with modern parts.
