i think you guys are missing the point slightly here, a quick description
40's - is the maximum choke size the carb will take
45's - same a gain but 45mm max chock etc etc

this doesn not mean you will be running a 40mm choke, or a 45mm in 45's


just for a little info i had a nasp cosworth in one of my old cars and ran that on 48's but the choke size was 46mm.

the thing with bike carbs are that alot of them have variable venturi's giving better fuel control over more load area than a fixed venturi, variable is more precise, you dont get that with the traditional 40's etc.
go with the bike carbs.