HHO? What is that?
Tony, the entire car is set up for drinking gas like a wino out of weekend lock up. The biggest reason your mileage is in the toliet is the cam. Second was the Holley styled carb set up richly.
IF you wanna do the power tour without the power, install a small cam. Something close to a 2bbl cam. Chrysler advertised it at something like 252* w/.410 lift. Put the 2.76 gears back in. Keep the Edel. 500, though I'd recomend the 600.
IF you have stock 1.88 valved heads, swap them on.