I had adjustable Konis put in my former car ('70 New Yorker 4d HT). They used to be THE shock to have back in the day. My old man's car had them too, 40 some years ago. You can still find NOS pieces of the rear end Konis (# 82-1400 or #82-1787, both will do.) Front shocks (#80-1742) are obsolete nowadays, so I had a pair custom made out of Porsche 356 Koni shocks (#80-1011). To my surprise, they had (according to a period Koni catalogue) the same dimensions, and almost the same compression & decompression values as the original C-body shock. I picked up some info on the Koni shocks out of that catalogue, too, for comparison. I also had the KYB tested in a shock dyno, just to find out how it was. Here's the data:
Koni # 80-1742: 168 lbs bounce - 449 lbs rebounce (OE replacement, heavy duty shock. Obsolete.)
Koni # 80-1011: 168 lbs bounce - 460 lbs rebounce (Porsche 356 shock, in production.)
Koni # 80-2553: 168 lbs bounce - 483 lbs rebounce (The Isuzu Pick Up Truck shock, Koni Heavy Track-series. Non-adjustable.)
Koni # 80-1914: 236 lbs bounce - 225 lbs rebounce (The '67-'69 Camaro shock, which will fit in, but is both too hard & too weak.)
KYB Gas-A-Just: 202 lbs bounce - 225 lbs rebounce (Like the Camaro shock: too hard going down, and too weak in actual shock absorbing.)
From my experience, the Koni is a superior shock absorber for a C-body. The Konis feel very soft first, but firm when you start to push harder. You can read that from the data, too. The Bilstein is probably very much like the Koni. I have no experience on it, however. - I just recently purchased a pair of NOS Chrysler Oriflow front shocks. I intend to have them tested in the dyno. Just to find out how they are. Will post the results later.