No mystery what the problem was anymore. The clutch disk was completely shot. The entire disc was nearing end of life, but there was also a section of friction material that had come loose which I found at the bottom of the bellhousing...so that could certainly cause a vibration.
Now I'm looking for replacement kits. The pressure plate/disc were 11.5" 10-spline long-style.
From reading a bunch of toploader posts on the forums I see fairly unanimous praise for David Kee, so I've been looking at his website. It seems he recommends McLeod clutches (in that they're the only brand on his site), however there are no 11.5" models (
Super Street Pro Clutch Kits). I do see 12" diaphragm style kits that mention they have the same bolt pattern as 11.5" long-style models.
Anyway, in the end I don't know what I'm looking for, so I'm open to suggestions on how to go about choosing a suitable/compatible replacement. I don't mind the 'heavy' clutch pedal, so I don't have an active desire to switch to a diaphragm style, but is there any reason not to?
If not, are the two 12" kits at the bottom of the page in the link above good replacement options? The difference between the two is in the 1-1/16" vs 1-3/8" measurement. Is that clutch disc hub thickness?
For reference the car puts out a little under 500 bhp.