Ok, I'll give it a try till the pros chime in... there is almost no difference- they are D-Port heads like any other. For example, my 68HO has #16 heads just like the base 400. In '69 they gave the 400HO/RAIII a special casting #, but not much difference otherwise. There are slight differences to the valvetrain, such as HD springs and lifters, and oil drippers inside the valve covers, but these are not special cast round port heads as with RAII or RAIV. From a collector's standpoint, they are most valuable if paired with a matching engine and/or car, otherwise, there is no reason to buy them individually. From a performance standpoint, any 400 can easily be made to HO or RAIII specs by buying the cam, Long Branches, and carb.
