Ben, I like your thinking and believe it would work except for the problem that the finished truck would be badly underpowered. If your 6 was working very hard just to carry that big old bronco around, you mileage will be as bad or worse than it is now. Fuel economy is very much affected by how much weifht you are trying to move. Sometimes a bigger engine working less hard will give better economy. Have you considered a 302 or doing some work on the 351M? It can be improved. DF