Max is what you are comfortable with, I know guys who will run her up to 1600+ on short bursts. Ive known guys who have pulled hard (trailer in tow) at 1400 all day long. Would I do it to mine? Dunno, its that comfortability factor you have to learn to live with. 1250 is a very safe number and is usually the one that everybody states as the limit.
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1250 is what International says is the sustained pre-turbo MAX. You can tow all day at 1250 and not hurt anything other then your pocket book from fuel milage.
To the OP, Check out the Tymar filter out, or an AFE stage 2. Your down pipe is fine, it will not effect your EGT's much at all going from a 4" to a 3.5". H2e kits designed to push stage 2-3 injectors come with a 3.5 down pipe.
Give us a bit more info on your set up. What are your Max EGT's? Where is your Pyro probe? What is your set up? Towing? Single cab? dually exc exc exc...