I'm curious what vacuum readings others are getting with their slants. My '64 Valiant convertible with a Super Six setup and 3.23 gears, indicates 15" @ 60mph and 1800 rpms. At speeds greater than that, it drops off drastically to where it only reads 10" @ 70 mph and 2400 rpms. It has a new distributor set at 6 degrees BTDC. Any more advance results in some pinging and starter kickback.

Other than the Super Six, the engine is stock and fairly high mileage, but gets 2,000 miles to a quart of oil, so it's still in pretty decent shape. The gas mileage around town is fine, but seems to be low (18 mpg) on trips.

How does that compare with your slants?

18mpg is realistic with your gearing, carburetion and speeds.

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Too many people who were born on third base actually believe they've hit a triple.

You might want to try switching to a staged two barrel. Your 64 motor has a mild cam so the staged two barrel might wwork well and improve your MPG.