I replaced my oil cooler for a new one, replaced the valve cover gaskets, intake manifold gaskets, throttle body gaskets, and ordered the Felpro gasket set for all the gaskets and it comes with the coolant rings with o-ring inside. Nothing else needed to be changed. I also cleaned the Throttle body, intake tuning valves, intake plenum, manifold and injector rail. Also replaced spark plugs...heeh. Just the regular stuff but nothing major like HCV lines or anything.
If you have all parts neccecary and newer spark plug and such is shouldn't take you but maybe 3-5 hours. You need to make sure ALL gaskets are tight and in place...also use the Black Permatex RTV sealant with GM part number on it to make sure the valve cover gasket corners, half moon circles, and 90 degree angles are sealed. Use it when you line the bottom of the oil cooler plate as well for added security. So far so good. 2 weeks and no problems. Took me 5 days off and on maybe like 6 hours total because i didn't have all the parts.
ooohhhhh. forgot to mention the process to clean your fluids!! You'll need 2 or 3 oil filters...2 or 3 sets of 6 quarts of 5W-30 or your preference of motor oil. Lots of distilled water and 1 or 2 gallons of radiator fluid to flush and fill. Your call but that's what i did and everthing came out beautiful. Right now I'm looking at xylene, toluene, and acetone mixtures in gasoline for MUCH better engine life AND MPG!
BTW good luck with everything. Takes forever to fix, but you'll be glad you did.
EDIT: You'll also NEED to get some SEAFOAM for your engine. It'll clean your engine up when you use the first set of motor oil to rinse all the contaminants out from the engine surfaces. Then add your second to cycle and rinse the engine. I used the 3rd set of oil for final top-off.