I have no clue how much hp it will handle. What I do know is that about 20 years ago I cleaned out a wrecking yard going out of business and got over 200 3N71B transmissions because no one else would buy them. I scrapped every piece because nothing sells in them. And every one of them were cherry inside.
Over the years I have probably junked in excess of 2000 of them. I have NEVER seen one with any internal damage.
The transmission you have came in some Granadas and other Ford cars from 77-80. I think they were bolted to inline six motors, but don't know for sure.
As far as I know, there is not one part in them, not even the bolts, that will fit a C4.
Basically, the 3N71B transmission is a Japanese copy of the C4, except it uses thrust bearings instead of thrust washers.