Yup, most electronic tachs out there trigger off the primary side of the coil. There are ones out there though that will trigger of the secondary, but the ones hemmings sells don't seem to be it. Also, every one I've seen required 12V.
A smattering: '53 Gold Flash '67 Royal Star '71 Rickman Metisse '40 Silver Star '37 Rudge Special sixtyseventy Lightboltrocket road racer...and many more.
I use a snowmobile tach off 12v but you require a tach with lead that clamps over on outside of high tension lead to plug so that each spark is caught. Most of the shop tach that are digital and inexpensive are like that. Steve