Rlzegers

Just did a 4 wheel brake job on the 1978 Ford F-350 2wd. Because the master cylinder gasket had swelled I tried to get a new one. It seems my master cylinder is an odd ball in that the casting, cap, and gasket are all shaped with the large radius on the front of the cylinder, at the reservior for the rear brakes. All the replacement parts I've been able to find are the opposite with the large radius at the rear of the master cylinder and the front being more square. Because of this the replacement gaskets won't fit. When I did the brake job I flushed the system and refilled with clean fluid. I want to keep it clean with a good gasket, but I can't find one. Can someone give me a source or an idea of how to get the right one? The truck was bought new so I know it's original. Thank you.

Ron