the outside of the slide is virtually impossible to get over - the choke you can. i've seen dell ortos going for 330 in the winter and with no offers. a 38mm from a 400 would be fine by the way - don't think you're losing out.....
FCR is the other way to go but by the time you've bought all the bits a £150 carb is £225. that isn't cheap is it!
regards
Taffy