How would you go about getting a DA revolver converted to be an SBR? Would that be an NFA request and a custom build from Ruger's custom shop, or can you do it with an existing handgun
If you have the revolver and the stock, then it is as easy, just submit the ATF form 1. If you want to buy it already assembled, then go to an FFL/SOT class 2 or 3 and purchase it on the ATF form 4.
In either case, obtain the ATF forms from the ATF home page and fill them out in duplicate, two sided. Send it in with form 5330.20, your finger prints, mug shot and a check for $200 to the address at the top of the ATF form. The ATF will not deny it as long as it is filled out properly. You also need to ensure that you live in a state where SBR's are legal.
If you do it yourself with an existing handgun, then the serial number used is the existing one on the revolver. There may be people who have done this on the http://www.subguns.com