Anything upto 20mm taken off the freehub body is fine, beyond that you will find there isnt enough space inside the freehub body to thread properly. The freehub will require extra support if you want the bearing to last, I made a bronze bush to take the place of the alloy spacer, a nice running fit on the bore in the freehub, keeps everything in line. Not a lot else to it really, those generation Hope hubs are agriculturally robust and will stand a lot of abuse.