After hunting around for a "real" period tach for my Volvo project over a few months, I decided to make something that would have the same look, and be functional and fairly inexpensive.
I wound up getting a VDO 52mm tach from a 240 sedan (says Volvo on the face) for more or less free, these can also be found on e-bay for $15-25. Most 240 owners want the large tach that replaces the clock, so these are around.
The housing is made from sch 80 PVC line - the OD is 2.375" - ID is around 2.062" and will fit the tach OD with a bit of sanding (the ID on my section was a little irregular).
I used some 3/8 all thread pipe (used in lamps) to attach this housing to a plate with small nuts (again from the lighting section of your hardware store), and added a small triangular Al. part to support the whole thing.
I ran the wiring thru the threaded pipe and machined tapers and grooves into the PVC to make it look more like the "smiths" instruments optional at the time on some Volvos (and standard on the 123GT) with a turned Al. plate in the back.
None of the above is really necessary BTW, just makes it look a bit more like it should.
The wiring is easy too, One "hot" - one ground - one wire from the coil neg. - and one for the light (I spliced into the heater control light for this).



Cheers!
OK