1957 Ford 640

Make sure your breathers on the rocker cover and oil filler are clean. If not they may pressurize the engine too much and force oil out the rear main seal and maybe the front seal. I would sure try a new pan gasket with sealer before I redid the rear main seal. According to what I have read, it is possible to put in the neoprene seal with the crankshaft in and presumably you can do it without splitting. Is the oil coming out by the back of the pan or out the hole in the bottom of the clutch area of the transmission case?

As far as value goes, are you really considering selling your Dad's old tractor, now that you have tracked it down? Some things are worth a lot more to a person for sentimental value than they ever would sell for on the open market. I doubt that I will ever sell my Dad's last tractor and am a little mad at my brother for not giving me first crack at it when he sold Dad's tractor that we had when I was a kid.