yo me again! the 40 or 50 refers to the percentage of the width in height of the sidewall so a 40 tyre will have a side wall height 40% the width of the tyre. i think thats right! so in theory a 40 should give you more clearance but not loads.
like i said in the other topic i'm afraid its got to be arch work to get the clearance, sorry! (but it'll be worth it!!!)