I knew a couple of guys (on-line) who used freighters; Both were Canadians. One was a moose hunter and used one with a ?10hp? to go back to his hunting area and, at least once, to transport himself, his hunting partner and a a quartered moose back out. I tried to find the site and thread, but it seems to have disappeared.
The other was a RCMP Constable, and he'd posted a few shots of local population hunters, one of which included a freighter.
I think, when Grumman sold their business to Marathon, the product changed. I believe they made shorter square sterns, and of thinner aluminum.
Here's one site I found, but it may be too general for you.
From what I'd seen and read, the Freighter was an industrial-strength craft, made to carry more than a thousand pounds of freight, as well as the passengers. I don't know what rating for horsepower they set.