I want to go from a 4" to 6" barrel and shroud.

This is a gun I bought to be a "truck gun" and know nothing about DWs.
Will any DW barrel and shroud fit?

Barrels from DWs are interchangable, no matter when the revolver was made.

See http://danwessonfirearms.com/ FAQs

I ended up going to one of the auction sites and found that my older gun took a different shroud than they had offered (cutoff on doifferent style is about 50,000 apparently), but that they were going to put a matching set-up out tomorrow for bid . . . Now it will just be a decision how much 2" more barrel is worth! I have never chronoed 4 inch vs 6 inch in the 357, has anyone else? I'm guesing 75-100 fps .
The gun out shoots either of my other 357s (Blackhawk and Model 66) even though every time I start shooting it, I have to get used to the feel (better than Smith when you finally get it settled in).
DWs's web site sure isn't a selling point for them, but maybe if they build a site in the future it will be worth visiting!