Autometer UltraLites match the brushed aluminum and lettering very well (except they are round, not sguare )
Some non-turbo Regals had the optional five pod dash. It included coolant temp and amperage gauages. My tach is from a Cutlass and as Jim mentioned, I mounted it to a clock delete plate and it's in the clocks housing. It was a lot of work. I still need to add some markings (been lazy lately).
I bet the same could be done with an Autometer tach with faceplate overlaid onto the clock. But I'd be to scared to cut open an expensive tach.
The square is only 3". A 2 5/8" should fit, but I would get the overall dimensions first. They might need a lot of room behind them.
I wonder if it's possible to call Autometer and just buy a faceplate? That's assuming they have one that's scaled the same as the cutlass tach. (just thinking out loud )