You could do it....
All you need is a PK6 012, an adapter plate, an hydraulic clutch slave clinder, 2 CV joints ( inner that mate up to the PK6 box ) some high pressure pipe and a clutch pedal that has a master cylinder attached.
Bolt the plate onto the block, make a template of the gearbox bellhousing, drill the nescessary holes - bolt it on ( make sure the main drive from the box aligns and fits into the flywheel on the F4R lump. Use the 172 clutch.
Fabricate a bracket for the clutch pedal / master cylinder bolt it up and then attach the pipework going to the hydraulic slave cylinder. Make a reservoir and bleed the system up.
Then cut the ends of your 172 shafts ( inner sections ) measure, balance and attach them to the CV joints that slot into the PK6 box. Some PK6 boxes have speedo drives and some dont ( ABS models ) so if your car is a cup or a full fat you can be sorted either way.