Actually to reduce RPM's relative to MPH, you want to install a lower ratio. I believe for your car you would need to reduce from 4.2 or 3.9 to 3.7. This would decrease your RPM,s by 100-300.
These units can be costly $100+ because they were only used in the '79 and '80 Midgets. They do come up on Ebay and a relatively easy swap.
I would check to what your rear is on the side of the pumpkin. A 3.9 is stamped 39-10. If your has this stamping you would only get a 100 rpm benefit, hardly worth it. If it stamped with the 4.2 stamping which I do not recall (42-12?) then this swap would get you 300 rpms and is probably worth it.
There have been other posts regarding these stampings, so I am sure others will have more info.
Patrick