275-40-17 or even 285-40-17 fits with some fender lip adjustment. George D. has 285s on his SWB car, on 9-1/2" rims. Wayne Ing had 315-35-17s jammed on 8-1/2" wheels a few years back as an experiment, they sort of fit, but not really recommended especially on a rim that's at least 2 inches too narrow !!
Who's making a K-member for an MN-12 car ? Are you talking about a tubular one like the ones for the Stang boys ? If so, who's building / selling those ?