Re: sea nymph max hp?
My extensive collection of literature does not include the '77 SeaNymph model year. In the 1980's the deluxe models were in the "SC" series. Based on your description, and my old catalogs, I would guess it would be rated at 35-40 HP for a tiller model.
Based on my experience with s SeaNymph 14R, I would stick at or below 35. The older SeaNymphs were built with marginal structural integrity. Certainly a V-4 would be a dangerous combination.