So why not convert the stock stem to threadless? I'm envisioning using the Sheldon Brown method with a pinch bolt collar to hold the preload tension, but also using spacers to get the stack height up tight to the riser when it's clamped on for extra insurance.



Any thoughts? Anyone already try it?[/QUOTE]

It's not a good idea. A collar clamped on the steerer will not provide the micro-adjustment you need to hold a good headset adjustment setting. The threaded headset will keep a good adjustment, and the threadless headset really needs the star-fangled nut in the steerer tube to do the same.
Even if you tapped it in place and clamped it down as firmly as you could, it wouldn't take many miles before it slipped enough to have the headset loosen.
I know because a bike I made relies on this type of headset arrangement. I had to make a pinch-bolt collar that incorporates 2 set-screws to keep it there. That solved my issue; the collar does not move any more. But I would not want to take it apart unless necessary for an overhaul or replacement.