OP, I tried to find a solution to your problem, but being half a world away things are done differently there. As a professional sitting in the drivers seat of an auto shop for many years, I could not charge a customer to make a critical, high failure rate part, just work. Instead the solution would be to return the parts in hand, and purchase the proper hubs, even if that means purchasing the sealed units, all due to liability. If a bearing failed for whatever reason, say launching the tire/hub assembly into traffic, and causing an injury/damaging accident, the installer could be deemed liable.
If you can't find a better solution and want to try to cut them down, it will take a measuring device, a permanent marker, some type of holding device aka vise, and a 4-5" grinder with a cut off wheel. Clean the grease off or the Sharpie won't work. Measure how much you want to cut off and set the pen height to that spec. Rotate the old bearing over the pen marking it all the way around. Don your personal protection and start cutting, checking to make sure you are on track. Once the cut is completed you can grind off any high spots if needed. Verify with a combination square that it is all good, and retouch any areas that are not. If they are not 100% operational, throw them away and buy the correct hubs, which is the right way to do the job, and what I would do for my own trailer.