I recently cut up the the stock gauge cluster to install a wideband O2 gauge in place of the OEM coolant temp gauge. I first saw the idea on Max Cooper's page:
http://www.maxcooper.com/rx7/how-to/...ges/index.html
I removed the coolant gauge because it's been non-functional for a few years. I made a digital gauge using the coolant sensor in the rear iron for an instrumentation class. It's mounted behind a smoked plastic lens covering the warning lights on the left side of the cluster.
The lighting on the O2 gauge is too bright at nighttime, I may adjust this eventually. Aside from that I'm pretty pleased with how it turned out.