The 67 and older have a smaller crank shaft register.Thus flywheel and torque converter snout sizes differ from the 68 and later.
Around 1976 they went to cast crankshaft and the block/crank is different.1980 was the last year for a solid cam.
Not sure but around 1974 the head changed and the spark plug tubes were no longer used.The new head is commonly called "Peanut Plug" because of the 5/8 hex plug.
