[1968firebird455]

Post a picture of the input shaft of the transmission. I agree with Tom S. I replace the pilot and throwout bearings every time.
I have had the best luck with the centerforce PN: N1716 throwout bearing. I always look at the front bearing retainer that the throwout rides on. This should be perfectly smooth. If not, replace it. Next slide the throwout bearing over the top and you should have very minimal 'rock'. Most clutch chatter I have encountered is from excessive bearing to retainer clearance. Next wiggle the input shaft. I have seen excessive play in this as which is either a bad input shaft bearing or the case itself is bad. Next, go to the bellhousing and make sure the pivot ball is 4.75" down from the mounting flange. Make sure the pivot ball doesn't have excessive wear. It's too late now to see how it was installed but I have also seen throwout bearings install on the clutch fork wrong.