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The pic on the top is a standard ralley gauges. Notice that the big round one has fuel, and temp gauge and the 2 small ones has alt and oil and they're blank underneath. So if you cut the hole for a tach, how are you going to hook-up the fuel and temp gauge

look at pic 2 the small ones have oil alt fuel temp



As Nacho mentioned above, it can be done, but you still need the gauge faces (the black plastic part that covers the needle attach point and mechanical workings). The gauge backing plates (the metal part behind the needle itself) would need to be painted and decaled.

I havent looked at the internals, but my guess is that the internal gauge workings are similar, so that they can be reloated to the appropriate new locations on the cluster frame, (reverse wire one for proper movement).

Probably be easier to find someone parting out a clock/tach cluster to get what you need. I sold a clock/tach cluster frame with two good gauges on it last year on eBay for $15. (No bezel and bad plastic.) So the stuff is out there.