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I honestly have no idea (haven't calculated) on MPG, would assume 8, used to get around 13 with the 400, I have some figures on that, but that was before they added the ethanol to the gas.
Your answer is that you have a 1978 360 that is sucked in low compression. The 318 had 160 hp and the 36o may have had 180 hp...they smogged dogged these Dodges back then. Higher compression pistons and get rid of the lean burn ignition.