(snip)"...because I don't feel it in the car, just the shifter."
Well....DUH!
You don't feel imbalance "in the car"
because the powertrain is soft mounted.
As someone who seriously-flamed ZR51 Performance (the ZFDoc) for making statements which were...
"...in excess of his actual knowledge",
...I am astonished that you didn't know that.
Some basics for you
1) When you change flywheels on 89-96 L98, LT1 and LT4 engines you need to deal with the weights (those pins in the flywheel) in the manner that "c4c5Specialist" has suggested. Information on those pins is, also, in the 90-95 FSMs but, strangely, was omitted in the 89 and 96 books.
2) When you change to an aftermarket clutch, even units which are claimed to be "neutral balance" by their manufacturers, be prepared for imbalance. I never install an aftermarket clutch (ie: Fidanza, Centerforce, McLeod, etc) without having its balance checked.
3) Flywheel/clutch imbalance will peak between 3000 and 4000 rpm. The reason you feel it most in that range is a) humans' sense of touch/feel tends to be more sensitive to those frequencies and b) the car's structure tends to resonate at those frequencies.
4) The reason imbalance might be felt through the shifter (a buzzing sensation) but not through your butt is that while your imbalance exists, it's not bad enough to get the car's structure to vibrate. Because of the damping effect of hydraulic and rubber powertrain mounts, the amplitude of the vibration must be higher to be felt "in the car" than it needs to be to be felt just "through the shifter". I recently had a similar, but worse, problem with an aftermarket clutch in a C5. The manufacturer had made an error in its balance work leaving the clutch 40-grams out of balance, which is a ton of imbalance. Yep, you could feel it in the shifter for sure and, between 2500 and 4000 rpm, you could feel it and even hear it in the car's structure.
Now, back to ZR51 Performance, the ZFDoc...today, there are few engineers left at GM Powertrain who worked on the S6-40 and not many at ZF Industries, but there is a small group of rebuilders around the country who specialize in the transmission, and one is ZR51 Performance. Through the technical writing and service work I've done with the S6-40, I became familiar with Bill Boudreau, the "ZFDoc," about a dozen years ago. While it is true that Bill is not an expert writer or web designer, he is an expert on the ZF S6-40. The vast majority of information on his web site, while it may have misspellings or improper editing, from a technical standpoint, is accurate.
I am qualified to assess the accuracy of information on that site because few people, outside of those engineers from ZF and GM who developed the transmission during the late 80s and early 90s, know more about the ZF S6-40 than myself. I covered the engineering of the ZF six-speed extensively for magazines in the early 1990s. I did two long interview sessions in Michigan with engineers at GM Powertrain and at ZF Industries during 1992 with resulted in articles in
"Corvette Quarterly" and
"Vette" magazines. I've owned and maintained not one, but two cars with ZFs in them.
In 2000, Bill and I worked on an article about ZF service work and during the preparation of that article, together we disassembled, photographed and reassembled one of my ZFs along with several others he had in his shop. All this experience with the ZF S6-40 allows me to comment upon that web site's content.
ZR51 Performance, the "ZFDoc," is one of the best choices in ZF S6-40 service and rebuilding. In my opinion, the slanderous statements expressed by "dmg" that Bill Boudreau makes statements which are...
"....in excess of his actual knowledge"
...and that he is attempting to misrepresent transmission problems to unsuspecting ZR-1 owners with the intent to profit fraudulently, are so much shear speculation on "dmg's" part that they are laughable.
"Dmg" has
no basis in fact to make such statements. What he posted about the ZFDoc is pure B.S.
The flywheel I replaced on my 90 L98 had the holes , but no pins in any. So I didn't put any in. I have never noticed any vibration, and is smoother than it was before clutch replacement. 
