Hey MazdaRx
Sure the 2nd Gen B12 shock will fit fine in a 2nd Gen B6. It IS a little longer than stock, and will raise the rear slightly, but this is a common upgrade for many B6ers. (I have a stock B12 shock in my B6 now).
The other benefit of the B12 shock, is it normally comes with a slightly stiffer spring than a std B6. I don't know if this is true for the Progressive 420 or not, but is generally true.
Both Progressive & Works Performace shocks would be a decent upgrade to a standard B6 shock. Ohlins, are an excellent upgrade, but usually quite expensive.
You can also consider a 1st gen "Busa" shock, which is longer yet than the B12 shock. You will want to firm up the front forks with stiffer springs to compensate. You can also change to a taller 120-70 series front tire from the stock 120-60, which would raise the front a bit as well.