Isx 870 with 1.2 million miles 05 international

[Izzyrod52]

I'm brand new here but I'm seriously impressed by how much info and knowledge there is for these engines. I'm looking to get started with major TLC on my truck. It's a 2005 international 9400i eagle with a cummins isx 400st cm870 bone stock at just under 1.2 million miles. 2.64 rears and a 10 speed. It leaks oil from the oil cooler gasket and now from the oil pan gasket as well. Front and back covers leak as well. I'm definitely due for inframe and out of frame overhauls so I'm just here looking for any advice on how to get started here. I will be ordering parts and doing all the wrench turning at home. Considered selling or trading it but I feel that the truck is worth putting the time and effort into rebuilding. I change oil every 10,000 miles and that's basically all I've ever had to do. I'm blown away at this engine still running so well and strong but am well aware that the time has come for major work to be done. What do y'all think?