Early sixties merc 400 and 500 differences??

[Doug Redinger]

Can anyone tell me what the differences were between an old merc 400 and an old merc 500?? The block is the same part # so I am assuming that the intake side of the crankcases are the same. The exhaust baffle plate #'s are the same as well, so I don't think there is any difference in that area. The only difference I can really see is the carbs are different part #'s?? The 400 was rated at 45 h.p. and the 500 was rated at 50 h.p. Could I increase the power of an old 400 by adding 500 carbs to it??