Thermostat is on the driver side, behind Idler pulley above a/c pump. And yes it will be the straight water neck assembly. Passenger side is just for the free flowing of coolant.

 

As goodbrian47 says, that is not where the thermostat is on a 3.5 engine. Find where the lower radiator hose is connected at the radiator. Follow it to where it connects to the engine block. That is your thermostat. The first thermostat ($46?) would probably have worked. This might be a good time to invest in an OEM part. You will see, when you go to replace it, that you won't want to have to do the job again for a while.

 

Thanks guys. Just when I though GM was over the bafflingly bad ideas (C4 corvettes as stylish as a doorstop, reverse flow cooling, optispark distributor, etc) they manage to turn a 10 minute job into a nightmare. And how does it even work, being on the inlet?

 

It shouldn't matter whether the thermostat is in the outlet side or the inlet side. Either way, the control side of the thermostat will be towards the block. When the block heats the trapped water to the set temperature, the stat will open.

 

Great improvement in design, race inspired, I like it a lot. Go genuine, replace hose while you are at it. I use Peak Antifreeze (Not "Premix") that I mix 50/50 with distilled water. I also like the factory clamps over older screwed clamps.
In our first gen 3.5, we have a good effective cooling system that does it's job well for what it is designed for. I have had far, far worse.

 

You've already gotten some good answers. As others have mentioned, the thermostat is on the side of your engine. Below the battery and behind the alternator. If you've never changed one before suggest checking out a few YouTube videos for tips. Here's a link to one to get you started: