I see used (read:cores) toploaders at swap meets all the time for 3-500 bucks. You have to assume that these used gearboxes are just cores unless you know for a fact that they are in top shape. For instance, the "used" (it was the original) box in my fastback cost me $1000 to rebuild after I got it apart. Although it shifted fine when I got the car, I found a bent mainshaft and 2 gears in need of replacement.
If this is really the gearbox from this guy's k-code, then it is probably worth 1000-1200 to HIM. To anyone else, it is just a used gearbox. So...if you can get him up above 900, then I'd say sell it to him. As the others noted, the partial VIN is worthless to anyone else.
BTW, I thought all early (65-66), small block toploaders were wide ratio boxes. I could be wrong, but that is what I'd always heard. Not that it really matters in this particular discussion, but when did they start using close ratio gearsets in small block applications?