You can get those springs directly from Eaton/Detroit Spring... in stock height, 1,2, or 3 inch drop. I took my 3600 rear springs to the spring blacksmith recently and he set them up at 2.5" . I think it cost me $240 and they were completely disassembled and inspected during the process.
On a side note, If you are going to use a different rear axle, I am getting ready to measure my new axle setup for frame/driveshaft clearance, and there might be an issue with the center crossmember interfering with a one-piece driveshaft.... I might end up having to go with a custom two piece $$$ . Driveshaft guy says I need a 3.5" diameter shaft, as it's going to be about 60 inches long. At full bump stop I don't think a 3.5" one piece shaft is going to clear the crossmember. That would be a bummer.
I'll post my results here on the Hipo Shop forum.