That one will fit but my personal preference is for the Dickson type. I have a S1 size on my Wm250V and it is very rigid. I've never been keen on the ones fitted to Myfords, they always look a bit weedy but a lot of people use them so they can't be all bad.
(When I say it will fit, you will most likely need to modify either the base of the toolpost or the boss on the topslide) I went for re-machining the boss on the slide rather than machining the very hard toolpost)
Warco sell toolposts ready modified to fit the WM250, they are Dickson style and look to be reasonable quality, certainly on a par with the quality of the piston type.
Edited By Graeme W on 17/06/2015 20:25:33