What is the most I can mill a BB water pump housing? My elephant ears are 3/8" and I don't want to use a 3/8" crank pulley spacer because I don't have a lot of room to play with.

Can I take the whole 3/8" off the housing, or would I be safer taking 1/4" and using a 1/8" spacer on the crank?

Thanks guys!

Here's a picture of what I'm working with: