Well, since you've already put "a ton" of work into it, what's a little more time and money at this point-to fix it right?
It COULD simply be that the clutch facing is worn out................... Or that the pressure plate springs are weak-either from age overheating, and can no longer apply enough pressure to keep things from slipping; OR the flywheel or pressure plate could be warped from overheating; OR the flywheel could be rusty and uneven enough t hat the friction disc can't grip it well; OR the main shaft could have a burr on its splines or be rusty and not be allowing the full pressure of the pressure plate to be applied. OR the pressure plate cover may have been misaligned when installed; OR any combination of those. .....................................................................
But if its any comfort to you, the odds are good that its not been sitting anywhere near as long as since 1988-or the engine would have an excellent chance of being seized and at least some of intake and/or exhaust valves would likely be sticking. (ALSO an automatic trans in this vehicle is a known problem child-and very expensive to rebuild. ) .........................................................................
All in all, if the clutch cover WAS installed correctly, and even if new parts were used, when vehicle sits for a very long time, the odds are pretty strong that the fly wheel friction face will be coated with surface rust as a MINIMUM and if sittiong long enough, more likely pitted too. ............... If its ONLY surface rust, that will often wear off quickly, BUT while that's happening the friction surface of the flywheel's friction surface is getting burnished/polished, and If that's the case, even a new clutch friction disc wouldn't be able to grab as well as if it was properly machined-plus the friction disc is getting rust embedded into its surface, making it less effective too.
Bottom line is that without taking a look you're just speculating.
Mine's a later year automatic with 2.9L, but if the starter on yours still mounts with a big hole through the bellhousing that's your inspection window to check for a shredded friction disc and (with a small inspection mirror) the condition of the flywheel face for rust.
If I didn't like what I'd found, I'd figure the cost for refacing or replacing the flywheel, and installing a FULL clutch kit.
Being that its a 1984,replacing the clutch cylinder wouldn't be a bad idea either.
