seymour,
use a zipdisc to cut a slot in the end of the tapered stud. now, with the jack applying pressure to the joint, use a torch and heat the nut to a dull cherry, and remove it with an open end wrench while holding the stud stationary with a screwdriver in the slot you just cut. if the clearance is tight, you can jam the screwdriver into the slot from the side.
tapered sockets will not wear out unless the assembly works loose and the stud works back and forth in the hole. it is possible, and i have had this done many times before, for a machine shop to repair the bore-hole by pressing in a bushing and reaming it back out to the correct taper. in the case of a $300 spindle, the cost savings of this repair are significant. the k-frame will have to be removed and stripped of all removable parts before sending to the machine shop, so that it can be held in a fixture for this machine work. in some cases oversize tapered studs are available as service replacements, but i cannot comment on your car directly. if the hole is indeed worn oversize, and the stud has to be pulled upward an excessive amount to become tight again, the boot might become shredded from interference or the joint itself may interfere against the bracket.
an alternate solution you may want to explore is using a repair polymer like belzona, and using their release coating on your *new* idler arm stud to restore the taper on the k-frame bracket. i do not have direct experience with this type of repair, but if you explore their website you will find an archive of case histories where various formulas were used to solve very unique and demanding problems. it is interesting reading. start here to find a location near you:
http://www.belzona.com/locations.aspx
i hope this information is of use to you, and it is an honour to provide it to someone of your standing. best of luck to you.