Did you get this solved, Windxtasy?
If not, I suggest you get some Vernier calipers and measure the inside diameter of your "good" mast, vs. your 400 mast, to know the difference you're dealing with. Measure also the outside diameter of your adjusting collar. You can get calipers from general hardware stores for probably about $15. They are quite a useful tool for accurate measurement. Google 'em, so you'll know what to look for.
As some indication of ball-park figures, my (Ezzy) mast has an inside diameter of 32.6mm, and (Chinook) extension an outside diameter of 32mm, so there's 0.6mm clearance overall; ie pretty snug. These are for an RDM setup. The relative tolerances for an SDM should be somewhat the same.
Despite the "S" in SDM, I am not sure that there is a "standard". Is this a ploy by manufacturers to prevent us from interchanging components, or am I being cynical?
(RDM: Reduced Diameter Mast. SDM: Standard Diameter Mast)