Clutch
The grinding part can be hydraulics related. Ie not a proper clutch release. This usually accelerates the death of a clutch. An easy test here is to put it in first and press in the clutch. Rev to 4k without releasing the clutch. If the car moves, the clutch is not releasing completely and can lead to early clutch demise.
The big test for slipping is 5th gear 40 mph, dry smooth good road, a warm day and WOT. You will see the RPM jump on you and the acceleration feel sluggish with bursts. A good holding clutch with good traction should just bog and see the RPM stay low and do a slow climb.
However, the pilot bearing / bushing, pressure plate & throw out bearing seem to have failed now at 159,000 miles.
Some ot the shift issue your car has might be one of those instead of the clutch. Mine friends and sounds horrible going into gear seems like it's mostly the pilot bushing.
Your answer is in this thread, this thread is also posted at the top of this forum. And explains exactly what a shim is and what it does.
https://ls1tech.com/forums/manual-tr...need-shim.html
No one knows your car better than you do, it's time to read up and gain some knowledge







