I swapped a 350 into my M725 when I got it. It only gets 6.5 mph and maybe 7 mph on a very good day. Don't see many good days. It is not a radical motor, better pistons and a RV cam. Fresh build when I put it in. Has about 8,000 miles on it now. I am running a mechanical fuel pump. Carb is a marine Edelbrock. Carb might be part of the problem. Marine carbs don’t have any vacuum lines. Running manifold vacuum for HEI advance and brake booster. T98 / NP200 w/stock gears and 11.00 X 16 tires. Why is mileage so bad? Does the fuel need a return to the fuel tank?