[mrein3]

I have a spare 1971 Monte Carlo cluster (same as Chevelle SS) laying around. One cold winter evening many years ago I messed around with it and a spare truck cluster. The speedo bolts right in where the truck speedo goes. The only issues is the PRND1D2 window cut out in the Monte speedo - which probably isn't cut out in a straight stick Monte speedo.

My Monte cluster was an idiot light cluster so it had a clock instead of a tach. But the tach is the same size as the clock. And since they are all the same size as each other, you could do something like Monte/Chevelle SS tach in the truck fuel gauge hole, aftermarket or other Chevrolet fuel gauge in the center truck hole, and leave the speedo where it is.

For my truck I put an aftermarket tach in the tach hole and a clock in the open hole next to the steering wheel.

Just thinking out loud...