There's really no way of doing it and using the existing cluster. I've tried. The problem is that the stock gauges are not round, they're flat on the bottom. making the gauges less than 2 1/16 overall height, somewheres under 2" IIRC A 2 1/16" gauge is larger than that at the bezel as they're designed to fit into a hole measuring 2 1/16". The only way it might be possible would be to take a 2 1/16" gauge and completely hack it up and remove it from the case, but that's alot of work and if you kill the gauge, no warranty.
Plus be aware that most aftermarket temp gauges us a 1/4" NPT fitting for the sensor, and the 4.0 head was designed for only a 1/8" NPT fitting. I haven't tried but i doubt there is sufficient clearance for a 1/4" fitting.
Barnett performance does make a 4 gauge panel that is a direct bolt in and works with 2 5/8" gauges. However you can get only 4 gauges in there, and 2 5/8" speedo's are not easy to come by.
Dingo