I'm toward a cable about to go or trouble with the throwout bearing. However, just to toss out another idea, it could be a large chunk of the clutch lining coming loose from the disc if it is in really bad shape, but I'd expect more slippage before you hit that point. Pull off the inspection plate (two 10mm bolts at the bottom front of the bellhousing) and see what you find there.

If/when you pull the trans, do get a trans jack. That 86 trans is one of the heaviest trannys I've ever had to deal with.

You also might want to start hosing down the bolts that hold the crossmember to the frame with penetrating oil right away. I broke 2 out of 8, and stripped the threads off two more.