I have had to narrow a couple of secondhand bogies (picked up dirt cheap at Steam in Beds) to fit to an IMR coach kit I'm working on. The first photo shows the sort of reduction needed -about 20mm in total. I removed the electrical pickups, unclipped the leaf springs and sanded the axleboxes back; then shortened the mounting lugs and the bearings: I just got away with not having to cut the axles. It needed some brass shims to keep the frames vertical, then new axlebox covers and a couple of bushes completed the work; installation required prototypical cutting of pockets in the back of the running boards, Proper testing awaits, but a quick push around my 4ft radius curves would suggest the surgery has been successful.