IT'S IN GEAR! sort-of. 1972 Ford Mach 1 Transmission Will Not Fully Engage. Don't Know Why. Linkage? 

That big spring on the pedal assembly is for a stock long style or borg and beck style pressure plate, if you have the old school pressure plate you need that spring. If you are using a diaphragm style pressure plate you need to remove that spring or the clutch pedal will not come up. From the rest it just seems like you need to adjust the linkage. The movement of the fork seems normal as when you pull and push on it the throwout bearing is just riding on the input shaft. You don't want the throw out bearing up against the pressure plate unless you are pushing in the clutch pedal. Adjust the linkage to have 1/8" free play (with the spring disconnected from the clutch fork to the frame). When you are done reconnect the spring. That will pull the throw out bearing AWAY for the pressure plate so there is no undue wear. It seems that the adjustment you have has the throwout bearing too far from the pressure plate arms, so when you press the pedal it is not pushing the pressure plate arms enough.

Could be either of a few things: 1) the clutch linkage is not adjusted correctly. 2) pressure plate not fully seated against flywheel. 3) throw out bearing not fully on release arm

The 71-73 had a very stiff pedal, they wired the spring closed to ease the pedal for areas where traffic was heavy for stop and go and for going down the track….

Also whoever built the trans could have not put it together correctly….