If you have that much coolant in the cylinders then I'd bet the UIM has cracked and filled them up. This will be evident once you remove the UIM. You will probabaly see Dexcool in the LIM everywhere.
Here'* what you need once you get the coolant out of the cylinders...
If you scroll all the way down to the bottom of that link, it also shows the Aluminum LOWER gasket set that is needed.
Get some GM gray sealant or black RTV as well. 6 new plugs. 7 quarts of cheap oil and a cheap filter, then 6 quarts of quality oil and a quality filter.
From there it'* just nuts and bolts and a few o-rings. Probabaly can be done in a weekend for a first-timer such as yourself. Also, since you have to remove the TB to change out the UIM, clean the entire TB including the blade inside. It will be covered in carbon. Make sure you remove all the electronics from it (IAC, TPS, MAF) first. Then clean the MAF w/ approved cleaner as well as it is probably dirty by now. MAF cleaning instructions can be found on here I'm sure. CRC sells MAF cleaning fluid at most Autozone/Oreillys. And when removing the LIM, don't forget there is a hidden bolt in the pass. side rear corner of the LIM, and another in the driver side front corner. These will most likely be covered by a pool of oil and not noticed right away until you try and pry the LIM off and realize it'* not moving, lol...
The reason for the 7 quarts of cheap oil is to use 1 of them to wash the lifter valley out while you have the LIM off. Then once you reassemble everything you change the oil BEFORE you start the car using the other 6 quarts of cheap oil/filter. Idle for a few minutes then change the oil again, this time using the good oil/filter. This will ensure that all the coolant is out of where it doesn't belong (your bearings).
Then you just have to fight to bleed all the air from the cooling system and call it done! And you just saved yourself $400.