You'll have to thank Bob Parmenter for this answer, which cleared the whole thing up for me.

When you weld one piece of steel to another piece of steel, the joint must be clean and free of foreign material to make a good weld.

When tetraethyl lead was added to gasoline up through 1971, it acted as a foreign material between the valve face and the valve seat, effectively "dirtying-up" the joint so that no welding could take place.

Current pump fuels have no lead in them, so there is nothing in the fuel to prevent the valve and seat from doing a little welding everytime they come together. This doesn't happen quickly, but rather over a period of time, with a little spot weld here and a little spot weld there which then effectively tears a little of the iron from the valve seat until the seat recesses down into the combustion chamber.

The question would be whether it's less expensive to bolt on '72-up heads or have seats installed in your heads.

Some other members on this board are much more familiar with FE motors and can tell you is there is a problem with fitment of the later heads, but as far as I know, an FE is an FE.