Bypass the fuel tanks and see if it will run off of a 5 gallon can. I'm betting that a line is allowing it to suck air somewhere. I would not want to swap to a 351, might as well go big with a 460 since the 351 and 460 will get the same mpg. Also don't forget that you will need weird hoses to connect a 351 to the diesel radiator, the diesel rad has much larger connections than the 351 does so you will need to neck the hoses up or have a radiator shop modify the diesel rad to fit the smaller hoses.