I'm not throwing stones here by any means, but too many people make removing the oil cooler WAY more trouble than it could ever possibly be!
The entire job can be done from under the truck with no mount removal, jacking up [or off
] and accomplished in under 1 hr.
Use a spot of spray paint to index the cooler bundle---this "should" not matter but it will help keep any particles not flushed out when rinsed from flowing thru the engine!
Drain coolant from both radiator and block drains, remove oil filter and filter header by using a long angled prybar for purchase against the web of the exh mnfld-----this may remove only the header or it will loosen and remove the cooler bundle as well.
Flush out the bundle from both directions using solvent and allow to air dry after using some scotchbrite to polish the seal areas of both headers and the bundle.
Removing the front header is optional as long as the gasket is intact, you only need to clean the seal ring areas.
I don't like using Vaseline due to it's ability to crystalize at operating temps.
You can buy a small tube of siliconised plumbers grease from any home center and that is what I use to liberally coat the seal rings and headers and cooler bundle with.
Install the seal rings ON THE BUNDLE!!!!!!!! not in the bores!!!!!!
Install is simply aligning the index on the bundle and starting it in the front header, then apply a film of grease on the header gasket and once again use the pry bar to press the bundle into the header and align the filter header bolts. Prime the filter and fill coolant---REMEMBER to bleed the air from the cooling system by loosing the clamp on one of the heater hoses at the fire wall to purge the air!
I know---there is supposed to be a ball in the t/stat hsg, but using the loose hose clamp on the highest point in the system garentees getting all the air out
just my couple pennies