Pantera,
I'd agree that the '57-'60 models seem to attract a LOT of extra options and dealer accessories. I think '58 is the high water mark ('59 a close second). You mentioned 3-speed, but then said GR (grade retarder). That would make it the Turboglide trans, which "technically" does not shift at all. It works like that TurbineDrive trans in a '63 Riv. Never really shifts, just changes torque converter pitch (or perhaps it's done with internal cones..?). The "GR" gear is a gear that's not in use when starting out normally. It can be selected intentionally for compression braking or driving in the snow. Many of those transmissions have blown up from guys holding them in GR for fast acceleration, then throwing it into Drive. The trans doesn't like that much....
As far as value, I'm having a hard time keeping up with some of the craziness that's happening recently, but for a #1 well done car, especially a convt., I'd guess about $85K, but then, things could go over the top!
By the way, air cleaner elements are the same for early Pontiac 3x2, early Buick 2X4, '58-'61 348 3X2, and '62-'64 409 2X4. I "think" the 3x2 air cleaner base is being repopped, but not for the A/C car. And no '58-only starburst lid is being done. It might be a tough hunt.
Cheers
Verne