RE: dan wesson 357 mag
I was not aware that Dan Wesson Arms made pistols with interchangable cylinders. I also wasn't aware that they offered pistols chambered in .38 Special. I've had one of their .357 revolvers for over 25 years, and it's a great pistol. At the time, they cost half as much as a Colt Python, and mine was much more accurate than a friend's Python...
I suppose it's possible they made a pistol in .38 Special, and someone installed a barrel/shroud marked .357. I assume that the reason the .357's won't fit is because they're too long, right? You might want to take it to a gunsmith and have it inspected/measured, etc.