Thread jack... With Rockwells you will not need the doubler setup. Find a TH400 NP 205 and do the LoMax 3-1 conversion, it will be more than enough gearing, you save driveline length, and the cost will be close to the same as a doubler. JB conversions has everything you need including low cost 1410 32 spline yolks. Don't bother with 10 or 27 spline parts, if you are serious about running rocks don't try to scab together a questionable t-case. If you need a second opinion talk with Tim Hull, he and I both run rocks and find that uber gearing is not necessary.
I will sniff around and see if I can find a case.
Tim