The fuel flow of a 6/1 at 1/3 gallon/hour is 21 mL/minute, so once the fuel leaves the heater it's going to have some time to cool while it travels to and through the injection pump, then to the injector where it will be heated again by heat conducted from the cylinder head. Once the oil is warm, drawing through the fuel filter and injection pump should be easy. Even if the fuel cools in the high pressure line to the injector, it has enough presssure behind it to flow well. Obviously, insulating that line is a good idea. Running a hot water line against the high pressure line would insure that you maintain high fuel temperature at the injector, but that involves a water pump.
Quinn