A toolchanger sounds interesting, but may not be the best choice for the IH. It will be a lot of trouble to build or adapt one.
I would consider instead a power drawbar. They're actually very easy to build, or you can readily buy one that's ready out of the box. You're talking a few hundred bucks at most and it will dramatically speed up your manual tool changes. A lot of the older CNC's had no toolchangers. Consider machines like the Tree Journeyman 325. They did have power drawbars and other quick change systems, however.
If you are really interested in building a toolchanger, have a look at this Yahoo group:
http://groups.yahoo.com/group/minitoolchanger/messages
Incidentally, the guy that designed that toolchanger sells a nice set of plans for a power drawbar that's pretty cheap. I'll probably tackle it after I get a few of my other projects out of the way.
Best,
BW