Talked to Don Ferguson and his fix is time consuming. He does the work when building the engine. He did not remember which seal but it may have been a 460 Ford. He copies the seal seat of the bottom of the main cap then machines another complete one. Then machines out the GMC area and places the aluminum one in he has machined.
In other words he make an insert of the 460 seal. Then fits it to the machined opening in the GMC.
Works for him. Good luck
