Since you're in the Olds section, assuming Olds V8, the intake has through-bolts in the housing. As stated, drain a little out, maybe 1/2 gallon at most into a CLEAN container (so you can just put it back in), which should put you below the heater core level and just below the thermostat. Check by removing ONE of the bolts. If coolant starts dribbling out, retighten and drain a bit more. Once the bolt loosening shows no more coolant drain, you're good to go.
Double check your new thermostat in that it opens. Put it in hot water on the stove, suspended on a wire off a wrench that spans the pot, and bring it to a boil. If you have a temp gage, use that to give you an idea of when it starts to open. But if not, just make sure it starts to open prior to the water boiling. Sometimes they're garbage out of the package and don't open at all. Been there and done that. Don't sit the pellet directly on the bottom of the pan as it might heat up faster than the water.
Advise to use gasket sealant (non-hardening is my preference) on the fiber gasket for the housing, and clean the housing gasket surfaces well. Also, the CSM advises on the V8 bolts to use a non-hardening thread sealer on the bolt threads. They are through-bolts after all.
Those are 5/16-18 threads with a 1/2" head on those studs/bolts, so don't overtorque them (13 lbs/ft on V6, 19 lbs/ft on V8), and ensure you use a new piece of 3/4" hose on the bypass pipe. If you have cruise control on 84-up, you should have a stud style bolt that goes on the driver side for the cruise control actuator bracket.
Why couldn't you just do all of it at the same time instead of doing one and then the other?
