mount modifications

i have a pair of stock 73/74 b body bb spool mounts. i got them in a package deal with a bunch of other items. i was told that they are what you need to put a bb in a 73-76 a body. i thought that there was also a modification that you have to make to one side or the other on the k frame. can someone tell/show what mods have to be done? tia.