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The truck in my avatar is a F2 48 with 226 with 3 on the floor (that's how my uncle ordered it). Mine at this point is my every day driver. It's not really a question of what engine you use but if the 51 has the same rear end, mine is 4.86, it really starts to wind up the motor any where above about 40. These are not high speed trucks. You might start doing some research into swapping out the rear end for a F250 1972 back with 4.11 gearing.
My carb sometimes leaks. It's is ether crud from the gas tank keeping the float valve open or gas boiling out from getting hot. You could be just having issues with the float valve. You might check that out and make sure it is sealing. Depending on the age of your fuel pump, you might also replace the diaphragm. It needs to be compatible with todays fuel.