That makes since. I am currently using a flex fan on a 4" spacer as my current shroud is only 2.5" deep. It cools fine I am just worried about that huge spacer especially at high rpm's. If I get that OER shroud it is 4" deep I could go down to a 2.5" spacer. I would feel much safer with that. I wonder if that 2.5" deep shroud I have is from a 6 cylinder or something? It fits fine it's just too far away from the engine.68 originally used a short water pump while 69+ used long water pumps.
I believe the 69-72 fan shroud will fit your 68 but the fan blade might not be far enough into the shroud for maximum effectiveness unless you also add (or install a longer) spacer to move the fan forward.