super hard clutch pedal
Oh, very wise. What's your approach?
Diagnosis is my approach. That's the only approach, otherwise you're guessing. Sure I could say "you might try....." or you might try..." but I don't like guessing, especially with other people's money. I'm certainly not going to try to diagnose a physical clutch problem over the internet, so I gave the only correct answer. You (or someone if you're unable) will have to diagnose it.........or get lucky throwing parts at it. Whether you like that answer or agree with it is your dilemma, not mine, but that's what's going to have to happen.
So with that, lets break down what you're asking here, ok?
First, all we get in the OP is "65 Barracuda 360, 4 speed OD" and "American Powertrain slave cylinder" with some kinda unknown clutch set with the "larger 10.5" bellhousing".
Yet, somehow we're all supposed to know from this very detailed description what your problem is.
Finally, after you were asked by a member trying to help we still only have a brand name with the slave cylinder and master cylinder. No bore sizes, no bore strokes, just "oh I got the master cylinder that fit", and some kind of "maybe close to stock pressure plate"......"I don't actually know".
I realize you have limited knowledge of "what's there" and that some friends were trying to help you out in a time of need, but expecting us to whoop out a crystal ball with such little information is expecting a bit much. Then trying to get smart with me for what was probably the best response, considering the amount of information given. I cannot jump through the internet and diagnose your car, or I would be doing it for the many friends on here that I love helping. So I suggested that you try proper diagnosis. No, I was not being smart........but you were.
Look back over your question and how it was asked. Decide for yourself if you think you gave enough info the first time, with so many unknowns. Unfortunately, you don't know much more. That would help. At this point, we're kinda shootin blanks if you will at your problem.
Did this combination ever work correctly? You didn't include that tidbit, which would help narrow it down somewhat. So it could be lots of things. Yes, it might be a mismatch between the master cylinder and slave cylinder. It could be linkage binding and or rusted up. Since you didn't give a physical description, we don't know. It could be the slave cylinder or throwout bearing binding up. It could also be a hydraulic hose that's broken down from the inside. That's how they go bad. Does it have any hard line anywhere? Someone may have accidentally crimped it somewhere. Again, this physical description was left out.
My point is, that we shouldn't have to pull answers out of you like teeth, even if the answer is "I don't know".
Like I said, I was being serious with my diagnosis answer, because truthfully that's the only answer and what you're going to have to do if you want to zero in on the problem, instead of possibly needlessly replacing parts.