1994 Cadillac SLS OVER HEATING

Did you touch the upper and lower radiator hoses by chance. If it was an overheat caused by a stuck closed thermostat, then the hoses would have been cool the closer to the radiator, indicating no flow through the radiator. Were the cooling fans on? White smoke is not necessarily the only indication that you have a leaking head gasket, that is when it is at it's most severe state. A block test with a gas analyzer, for combustion gases in the cooling system is the most accurate.

If you added coolant, how much? If it hasn't overheated, then the theory of a stuck closed thermostat is not necessarily fact. If it was low on coolant, and you pressure test it and find no external leaks, monitor it and if it gets low again, it may be going internal. A few common places for leaks are the water pump, water pump cover, water pump housing(aka crossover), hose connections, the radiators can leak as well. If it has a black(phenolic) water pump cover, that has been updated to an aluminum one, because of the possibility of leaks. But like I say, I would not spend a bunch of money trying to fix coolant loss without having it block tested. Head gaskets, or more specifically the head bolt threads that degrade, are real common problems on a Northstar. To have that repaired professionally, it can run in the neighborhood of $3000.

Typicaly, a bypassed heater core will not account for much change in operating temperature. Usually when the core is bypassed, it is due to a leaking heater core, and that is the option to replacement.