I have a 6.5 turbo diesel and have done some work to increase HP. Installed a EGT guage and would like to know what the maximum safe temperature is.

It depends where you put the probe . It should be in the exhaust manifold for best results .Some companies put them after the turbo . If it's in the manifold it should max at 1250 degrees . After turbo is kind of not so acurate . Closer to the turbo the hotter in may be .The farther down the pipe the cooler it is . And it cools off quick farther away . If it is after turbo , I would say 1050-1150 max depending on how close to turbo it is .

tinkerboater,
Not sure about the 6.5 GM, but I am told that my Cummins will go to around 1300 degrees for SHORT(as in seconds) periods, and I would imagine that this temp would be lower for the new Duramax as it has a lot more aluminum in it. Aluminum melts around 1200 degrees in unalloyed form, iron at 2700 deg.
I've run mine up to 1350 with no ill effects(so far). If you're doing upgrades, the EGT gauge is a MUST, and a boost gage is also a good idea. I have the SPA Technique 206 electronic gage which I can highly recommend, it has a faster response time than analog gages, and it combines boost AND EGT in one gage. I got mine from Diesel Injection Service(not the one in Grand Rapids, although they might handle them)
http://www.dieselpage.com
http://spatechnique.com/index3.html S.P.A website


I currently have the TST 270 HP cam plate(I have an old 12 valve Cummins- easier to upgrade than those newfangled 24 valvers), which changes the wastegate so I can run 30 psi boost, and also changes the fuel cam to give her more juice, but I have to be judicious in my use of the accelerator pedal! I also added a 4" catback exhaust, and removed the cat. converter and silencer ring in the turbo, and went to a bigger air filter(NAPA 2790, or was it 2970?), which helped immensely in reducing EGT and let it breathe easier.
Good Luck,
Paul