bad oil cooler

[ktbtrucking]

Update:

When done fixing the truck after start up still had some fresh oil showing in water reservoir ( put new one) and was worried a bit. Oil looked fresh so assumed it is new oil because had some gray liquid only in my engine.
Drained just water reservoir about 5 times and after some 10h idling and 100miles oil stopped showing up.

Also decided to set an overhead after this repair.
It is cm870 and you have to only remove oil fill tube to bar an engine no need to remove fuel processor.
I made 500k on this truck and no adjustments were needed. The feeler gauge were really tight on exhaust and intake rockers.
I made it first time and it is really difficult to put feeler gauge between crosshead and rocker. I had to wobble crosshead a little to get a chance to put feeler gauge.
Truck has 1.15 mil miles. Had some wear/ lines on rocker rollers on exhaust and intake but looked good overall for me. Took some pictures might be helpful for others.