jd - if you want a M19 with a 6" bbl find one for sale and buy it. There are probably a lot of blue steel versions availalbe. If you can only find a 4" the call S&W and see if they will rebarrel the 4" to a 6" if they have the 6" bbl.
Be advised S&W will not consider rebarrelling a 38 caliber revolver with a 357 magnum caliber bbl - that is either illegal for them to do or against their company policy. I think most reputable gunsmiths would feel the same. Can you imagine someone picking up that revolver, trying to stuff a couple of 357s into the cylinder because the barrel says 357 magnum, and having the gun come apart in the face and hands - far too much liability.
Look for the Model 19, find one, and have S&W work it over for you - they do a great job. HTH. Mikey.