I don't know about what kind ( brand ), but as far as where the gauge (s ) should go, I personally like them easily readable, but non intrusive. How often do you use the CD player?
Google " 1 din gauge pack or pod " ... 52 mm fits in a 1 din space, and is a very common size. You can get 3 ( I think ) where the CD player now sits.
Not that this is what I'd recommend, but as an example of what gauges are available, colours and lighting options ..
http://www.ebay.com/itm/06-08-Mitsub...4e8267&vxp=mtr
Or, if you want and excuse to put the Panasonic factory Subaru stereo in a box for posterity, and put in an Android based double din stereo, there are integrated apps so that you see the gauges on screen, Torque,an android app, for example, hooks into OBD 2 and uses the info to display. It'll also talk to an android phone, if just occasional checking is all that's required. No stereo needed. Google says there's similar for OBD1 systems.
"Evil McGuyver idea".. Don't know if this is do-able.. but anyways.. The SVX has two coolant sensors, one for engine management, and one for the gauge on the dash. Now, as far as we all know, every SVX and many, many other cars, have a "dead zone" Now the question is, is the dead zone a product of the sensor, or the gauge itself? Either way, my idea would be to simply mount a factory coolant sensor in a sensor mounting block in the cooling line exiting the transmission. Run the 2 wires from the engine coolant temp sensor and the 2 wires back into the car and to a double throw toggle switch. run the output from the toggle switch back to the wiring harness connection for the engine temp sensor. Now, theoretically, you could toggle the dash gauge between trans temp and engine coolant temp. Maybe a spring loaded momentary switch ( hold for trans temp ). Although most temp gauges have a dead zone between 180 and 205, they seem accurate outside of that zone. Checking them against the "other" sensor via the OBD port may be helpful in interpreting the dash gauge readings.
That idea came to me while I was typing about the Bluetooth stuff, because, although cool looking, it didn't address your original post, which was a trans temp gauge. Yeah, I thought, too bad there's no way to get the trans temp signal into the wiring harness.. hey wait a minute! what if we... so that's where that idea came from. There may be a few reasons why it is absolutely not recommended, and I'm sure if there are, someone who knows why will kindly post them.. So Ummm... don't start on that just yet LOL.. But hey, ya never know, if it's a viable alternative.... It'd be one easy way to monitor transmission temp.