It's been my understanding that the 8 hole crank flanges were made to accommodate the Fluid Drive couplings. Generally you'd find them in the 230, and larger, engines. However, the 218 in my truck does have the 8 hole crank, because my truck has a Fluid Drive, with a standard 4 speed spur gear transmission.
From what I've read, over the years, the 8 hole crank flange is thicker. Probably to accommodate the heavier FD. This added thickness is accounted for in the flywheel mounting flange offset. If you attempt to interchange the 4 bolt and 8 bolt flywheels you may encounter improper starter spacing, as mentioned earlier. It may be wise to do some measuring before going too far down the 're-drill' rabbit hole.