Looking for info on 74-78 C4 trans for swapping

Hello Eli and welcome to the site!

As for the C4 being weak in the II, I don't think that's necessarily the case. To the best of my knowledge the only thing that was changed for them to be installed in a II was the bell housing and the corresponding flex plate and converter. Otherwise, the transmission is the same as what was installed in other cars and trucks from that era. So, ultimately, you really only need the bell, flex plate, and a converter to make it work for a Ranger swap. Now, given that you're looking for a II specific part, they aren't too cheap or easy to come by. I think locating the bell housing will be your biggest hurdle, otherwise you can pick up a C4 out of pretty much anything and make it work for your application. I also think that whoever was giving you this "information" that these transmissions are weaker than other C4s are basically parroting that the Mustang II is weak, that they are junk, or whatever other bias that a lot of people like to share that look down their noses on the II. Unless they are confusing the C4 with the RAD 4 speed, they are mistaken. So, grab a C4 from whatever, look for a Mustang II bell housing, you can order the Art Carr flex plate or find a stock piece, find a suitable converter, and build the transmission to match your combo and you're golden.