Thanks for the replies. The 3 piece valve seems the better choice.
I do intend to nest the items for storage, not to make a double-walled vessel.
Another "nesting" question. Would the 40 quart stainless and 40 quart aluminum Winware kettles nest? I can see that I need to allow >1.25" difference in radius to hold the fitting on the aluminum kettle (as it appears to be narrower and taller than the stainless).
Ideally, someone with both could tell me. Just about as ideally, someone could measure the circumerence of the interior & exterior of both kettles (and the height at which the handles are placed).