I believe that Real King Cobra has mentioned that 1 5/8 is as low as you can go with the k-66 profile. Any lower, and the ball will not rebound correctly. An exception is the Accufast cushion, which was designed with the intent of using a 1 1/2" rail core, but I don't think it is really a k-66 profile.
I recently restored an Olhausen 8' OS table and per RKC's feedback, I used k-66 profile cushions (Artemis Pool 66 to be exact). My nose height came out perfectly.
I'm pretty sure the Accufast cushions are just a slightly modified k-66 profile. The top profile is flatter so that the top of the cushion doesn't stick up higher than the top of the rail. You can see from my third pic that the swell at the top of the Artemis cushion is slightly higher than the surface of the rail...