How are you detecting slippage?

You should not be seeing any tranny slippage in 3rd gear and non T/H. If you have enough pedal to increase the torque enough to cause a slip, the torque converter will have unlocked, if it's even possible to lock it up in 3rd and non T/H with significant pedal input. This can only occur near readline RPMs, as the lockup speed and RPMs are there. The slippage you are seeing is probably the TC speed differential, which would be quite high in 3rd with power on. Meaning, it is an engine speed and tranny output differential, not actual clutch slippage.

If you are in 3rd and T/H, there should still be no detectible slippage. There just isn't enough torque at the 65 HP setting. In OD with a heavy load, however, it can slip with enough throttle and little acceleration, like on a grade, long overpass, or passing w/o a downshift. If you aren't towing, and the truck isn't loaded, I don't see how you could be slipping in all but OD up a long, steep grade.

That said, if the control/main filter (spin on) is near it's useful life, the pressures (flow) can decrease enough to cause slippage under certain conditions. At the same time, all auto trannies slip to some degree under load. IIRC, the TCM considers ~1% slippage completely normal. The stock TCM program is pretty sensitive to excessive slippage, so if you aren't limping, it's probably not something to be too alarmed about.

It is possible for your intake/exhaust upgrades to have increased the torque rise enough during a specific range to put it outside the envelope. I haven't seen it, but it's not impossible. My setup is very similar to yours, but I haven't seen a problem with over 113K on the clock, towing very heavy or empty.