The Hpi posts should work, if they are two short then use washers and shim them to be the proper height, if they are too long they use a dremel to cut them to the right height.
If you have to cut the flared portion completely off they I would cut them short enough to add a washer on that side to avoid breaking the graphite plate it mounts too.
If the posts are within a few millimeters then find someone with a bench grinder and remove slight amounts of material from the top and bottom to make the proper lenght..
Hope this helps.