1974 power wagon questions

I got 14.5 mpg with my power wagon with a 318 4-speed, 3.55 gears, just keep it below 65 mph on the highway.
I put a modified 360 mag in it and it still gets 14.5 mpg but it pulls way harder.
I accepted a deposit on it from my neighbor, and am already getting sellers remorse, and it has not left my driveway.

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