The Shure M78S is not a true mono cartridge. It is actually a stereo cartridge which has had its channels tied together with wire connectors that simply hook left to right channels in parallel. If your amplifier or control unit has a mono switch it will do the exact same thing. My method is to use a Shure M44 wired stereo with a second 3 mil stylus for 78's and switch the styli when I want to play 78's. This means that I don't have to readjust the stylus pressure to take into account the 78 RPM stylus. Both track at 3 grams. I then just push the mono button on my old Hafler preamp, and mono appears in both channels. Easy and slicker'n snot on a dooknob. (How's THAT for a gross simile?)
I expect that you could get a Shure S35C, or even an M97, and put the 78 RPM stylus in it when you wanted to play 78's, but the M97 would require readjusting the stylus pressure when you go to 78's. The S35C would play stereo, but you could switch the control unit to "mono" and cure it that way, or cobble an SPST switch and connect it between the left channel signal wire and the right channel signal wire at some accessible point on turntable, being careful to avoid hum sources. S35 and 78 stylus track at the same pressure, and both styli fit the S35C cartridge. This solution allows you to use the same turntable to play both mono and stereo records, and you will find that 78 RPM records have a lot of vertical rumble that appears when you play them in stereo, that goes away when you parallel the channels. The rig is much more flexible this way.
You could also use a different cartridge and put it on a new Garrard cartridge slide, but you might have to change stylus pressure to play 78's depending on cartridge.
And good luck from the old needle switcher,