Chevy made the front seal for the pan thicker in 1975 to make it easier to r&r the timing cover with the pan on.You can inerchange most left hand dip stick pans with each other if you use the right gasket,but the rods will hit some of the 265/283 pans when more than a 3 inch stroke is used.Most timing covers will inerchange except factory roller blocks,the gasket must match the pan.Bill.