The diesel tachometers that get a signal from the alternator are normal. The alternator has an extra terminal for a wire to go to the tach. Not all alternators will work, but if you go to a good starter generator and alternator repair shop they will understand completely and should set you up.
The other option is a tach that uses a sensor like an electronic speedometer. The sensor is most often mounted in the flywheel housing and reads from the ring gear. This is the version I will use on my truck, my flywheel housing has a boss ready to be drilled and tapped for the purpose.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-