3 questions about 6s

If you're looking for a junkyard type of build, the Holset HX35 would work well with a /6. The Cummins guys are always upgrading their stock turbos so you should be able to find a good one pretty cheap. The compressor is rated at 60 lbs/min so it should be fine in the 400-450 hp range while still being in it's acceptable efficiency range, but would be capable of 500 if pushed a little harder. This is a popular turbo with the 4 banger crowd and would work well on your /6.

As for the exhaust, the bigger the better. If you can do a single 3.5" that would be great, but for your combo a single 3" would do fine. Use a Magnaflow, Dynomax Ultra Flow, Borla XR-1, Hooker Max Flow, etc straight through type muffler.

Rotella T6 (5w-40) is a good oil. I personally run it in every engine I own.