I'm posting this here for future reference. These are the adjustable RPM switches.
MSD 8743 chip kit
... this is a kit with 5 different chips. Each chip is designed for a different RPM setting, in 200 RPM incremenets. It contains different chips for 3000, 3200, 3400, 3600, and 3800 RPMs.
You can buy these in lots of places, including O'Reillys
, SummitRacing, Amazon, etc.