Remove the old thermostat :

1. Drain the cooling system or siphon out enough coolant to get the level below the top of the intake manifold.
2. Remove the bolt holding the water neck in the manifold.
3. Remove the water neck from the manifold with a slight rocking motion while pulling up.
4. Pull up on the thermostat to remove it.

You should have two rubber seals for the new thermostat. The smaller one (not an O-ring) goes into the water neck. The O-ring goes into the groove on the outside of the water neck.

Install the new thermostat :

• Clean all surfaces on the water neck and manifold shiney clean.
• Install  the thermostat - pointed end up.
• Install new thermostat seal.

Important thing is that the pellet (brass cylinder about 3/8" in diameter faces the engine but I don't think it will assemble properly otherwise.

Install the water neck:
With all surfaces clean and liberal use of white grease, the neck should just push in by hand (I sometime push with a big screwdriver centered on the top. The bolt just holds it in place.
DO NOT try to pull the water neck down with the bolt. Torque the bolt to 10-12 lb-ft.

Note: a "wobble" 1/4" extension and a short 10mm 6 point socket make life a lot easier, the manifold flange is in exactly the wrong place for a straight down shot - obviously the engineer who designed it never had to remove one).

I use 180F Stant "SuperStats" p/n 59848 usually in stock at Pep Boyz (plug) & just put one in the White car. My observation is that the SuperStat will hold to about +/- 1 degree of the set point where with a conventional 'stat it is more like 5 degrees, usually on the high side. It matters to me.

O-ring (Fel-Pro 0427 or equivalent) is always changed if the neck comes off (e.g. when flushing the coolant system) and it has been more than a month or two since replaced (Keep several on hand). If the inner rubber seal seems good I generally reuse them since it will not cause an external leak.

BTW there is a TSB on that inner gasket. Seems without it enough coolant could flow past the thermostat to make getting heat a lengthy process. Also 180F thermostat will generate more than enough heat even at 0F (observed) and will warm up to operating temperature just as fast as any other..