i think you may have been overthinking that operation slightly, my old hookers sure didnt leave as much space as TTI for changing #5 plugs but it wasnt that bad,i remember two ways much easier than described.
1. put a sparkplugsocket on the plugg from sort of under the headertube and then just put an extension into the socket without seating the extension just using it as a slightly wobbling extension inbetwen the headertube and valvecover,worked great.
2. a cut down wrench under the headertube might have been alitle bent but it worked aswell.
just putting it out there incase someone else has the same problem
