At 66K, I'm willing to bet it Does Not have a blown Head Gasket.
Be very wary of ALL Dealers, and most independent shops, seems everyone thinks that if you can afford a Caddy, you can afford to sink big bucks on it.
Diagnosing ANY Cadillac cooling system issue as a "Blown Head Gasket" seems to be a disturbing trend among most mechanics.
I would start with the basics: ensure you have the sealant tabs/powder in the Dex-Cool, check the water pump belt tensioner for proper operation and tension, the water pump belt (the belts WILL stretch with age, causing it to slip, especially at higher speeds), the purge tank cap, make sure the purge line is clear, carefully check all your hoses, even a pinhole or loose clamp will hurt your cooling.
Check your cooling fans for proper operation.
Your radiator could possibly be clogged, check it for sufficient flow and back flush it if it does not flow freely, or even have it proffesionally cleaned by your local radiator shop.
You say you have exhaust in the coolant, this wouldn't by chance have been discovered by the infamous "Hydrocarbon" test would it? The hydrocarbon test is pure bunk! Heated coolant, by nature, produces hydrocarbons. The only true test for a blown head gasket (barring oil in coolant or vice-versa or steam steadily produced from the exhaust) is the cylinder pressure test. A search of this site will tell you how.
I speak from experience, having purchased 3 DeVilles this year alone, all with "blown head gaskets, all cheap, and only 1 of the 3 truly had a bad head gasket. ($1300 for a used engine and voila, a nice gift for my daughter), one (97) had a clogged radiator on a recent new crate engine, causing it to overheat at extended highway driving, the other (98) simply needed a new pressure cap as the coolant was overflowing at 230 degrees, causing a loss of coolant which resulted in overheating.
Hope you take the proper steps to check these things before you let someone take all your money, possibly needlessly.
I firmly believe that a large majority of Northstar cooling issues can be solved quite inexpensively, and that their reputation for "Blown Head Gaskets" is almost entirely due to rip-off artists and/ or ignorance.
Hope this helps before you shell out big bucks needlessly.