With that kind of tach where the sensing wire wraps around a spark plug wire, I'd think if you can set the tach up for an 8-cylinder 4-stroke engine, that'd be the same amount of ignition pulses as a 2-stroke 4-cylinder outboard engine.
If you wrapped the sensing wire around the coil wire, it'd be getting 4 times as many pulses and I reckon you'd have to set it for a 32-cylinder 4-stroke!
I don't think a car tach won't work 'cause you wouldn't have the right signal off the engine. On the old Inlines with dual-car-coil ignition systems, a car tach hooked to one of the coils, set on 6-cylinders, worked perfectly. But the Merc CDI ign like on your '68 65hp is a bit different.
BTW, there are a lot of inexpensive new and used marine 12-pole-alternator tachs on eBay, you might check that out. Tach sensing would be off either of the (2) yellow stator wires connected at the rectifier.
HTH..............ed
Edit: Here's a NOS VDO tach for under $20 shipped:
http://www.ebay.com/itm/271882058329
