Down pipe should be no problem with the cab on. I have a 4" one piece down pipe. Didnt have any issues putting it on with the cab in place.
However, i have a buddy with a lift. If you were doing it on the ground it could be a different story.
Could be oil leaking past the seal in the turbo also. That is what mine was doing earlier this year.
It did come and go some as well.
If you arent loosing coolant then it is a good possibility