I wrote a couple times about issues I am having with that 6-speed. One issue is that the trans gets stuck in gear, to the point I have to literally slam it out of gear. Once it comes out I hear a bang in the rear end. I learned that, say when I come to my gate, I de-clutch and try to pull out of gear but it sticks. I roll to the gate, holding the clutch in, apply brake. Hit the gate opener, as the gate is opening, I release the brake, but hold the clutch in, yet the car starts to move forward, and there is even a slight bump so it's not rolling, its moving under engine power, with the clutch depressed... That's why it sticks because the driveline is under pressure and the rear end bangs when I finally jerk it out of gear. Has anyone else experienced this? I'm baffled because the clutch is fully released, or so I thought..
Perhaps your clutch is worn and needs attention? How old is this car? By chance was the car on a slope when it was stuck in gear? I have had that happen and just assumed that gravity was working against the load on the trans gear.
MrMajestical - I think you're on the right track. If the clutch material itself is worn out, the clutch would slip when in gear maybe even not move the car. Since you mentioned that the car will still move forward even with the clutch pedal engaged (pushed to the floor) it sound like the clutch isn't disengaging all the way making it hard or impossible to move the shifter because there is still a load on the assembly. On a older 60's musclecar with a 4 speed and mechanical clutch linkage, it would most likely be the clutch linkage being out of adjustment. It's either that or something more serious with the internals of the transmission. I'm not familiar with hydraulic clutch linkage assemblies and if they can be adjusted.
For the clutch to not fully disengage even when you have pedal fully depressed, you need to look at things like a problem with the hydraulic system. Think of it like a brake system. What would you be looking at if you pressed your brake pedal and the brakes barely worked? Bad master cylinder? Air in the brake fluid line? Bad wheel cylinder/caliper? Your clutch system has the same operating principles. You might also want to physically inspect the mechanical parts like the push rod, slave and even the release fork and bearing to verify they're moving as much as they should when you push the pedal.