I'm not familiar with the 1500cc engine apart from spending hours getting a pair of SUs balanced on one years ago. Have you set the tappets, topped up the dashpots and so on?
If the rev counter is going up,but the speedo isn't,it can only be the clutch.
With a functioning drive system,as the engine turns faster,so must the wheels.
You say the clutch looks ok.When you unbolted the clutch cover,did all the diaphram spring fingers come up and touch the cover?
Also check for cracks in the diaphram spring,if there's a crack you will have reduced clamp load,and clutch will slip under load.
If the clutch assy checks out,I would suspect a hydraulic problem.
You could have a partialy siezed slave or master cylinder,or defective seal/spring in the master cylinder.
If the master cylinder piston doesn't return fully to the recovery position,it will prevent full release of pressure on the release bearing,and clutch will slip.
All the other problems suggested, overdrive, SU's etc would, have specific side effects, mostly the engine running rough or overdrive engaging on and off. If its reving up smoothly but not gaining speed it can only be the clutch.
Could you possibly describe the symptom more precisely. As it could well be something as simple as a dodgy earth. did it cough splutter, die slightly?
friction plate, pressure plate & springs, thrust bearing & associated mechanics, flywheel face, all form " the clutch"... need to check condition of all & that they work together.