For the release bearing, this shouldn’t be any different than any other year Mustang using a Windsor/Cleveland based engine. Here are some potential problems:
- Some release bearings require measuring the depth of the clutch fingers and installing spacers. If it is not coming out far enough that could be an issue. It may be coming out, but not far enough as it requires more spacers.
- The system may not be properly bled and still has air, in that case the release bearing will not function. They can be quite difficult to bleed. I had to reverse bleed mine—done by putting the output of the release bearing into a jar of hydraulic fluid and then vacuuming the master cylinder.
- Stiff pedal generally is due to an improperly sized clutch master cylinder OR improper geometry on the clutch pedal in relation to the master. The master cylinder bore versus the release bearing bore needs to be evaluated similar to gears. A small gear turns a large gear easily, where a large gear takes a lot of effort to turn a small gear. That said, a smaller bore clutch master will reduce pedal effort, but it also has to throw enough volume to get the proper release bearing action.
All that said, I would check your bleeding efforts first. Then make sure you get enough clutch release engagement/disengagement. If that is sufficient, but you still have a hard pedal effort—either a smaller bore master or a bellcrank system is needed for mechanical advantage.