My guess is the market for 6 cylinder tachs is not there, so though you may find a 4-cyl tach, I doubt you'll get much interest in swapping the 6. If I'm right, it can't really hurt for you to try the adjustment provided you don't mind the markings on the gauge face.

All tachs of this era I have seen are simple enough to make your own diagram, and all have a calibration (adjustment). Whether the adjustment is wide enough to include the 4 and 8 cylinder rates is easily determined by experiment. It helps to have a reference, like a hand-held tachometer to match it up. If the range isn't available on the adjustment, a resistor value may need to be changed, such as the 62K-ohm resistor in this "modern" 240 tach.


--
Art Benstein near Baltimore

If you lend someone $20 and never see that person again, it was probably worth it.