I think the 3500 engine is just the 3400 engine that was discontinued, which always had intake problems, and the 3900 I was told was the 3.8, but just with variable valve timing, and the 3500 is the 3400 with variable valve timing.
I'm looking for an 06 Monte with a 3.9. I have a 97 LeSabre with a 3800. One problem I had with the engine was the notorious plastic intake, but had it replaced with revised one.
The 3.5L V6 in the W body cars is not related to any other 3.5 motors of past years. It is part of what I've read "value added" engine line up. It's all new so they say, but it sounds like a 3.1L or 3.4L that was in mid sized cars in the '90s and '00s.
This is a great looking car too. The headlight design is very unique, and the car has a hint of its muscle car history in the styling.
The 3900 is just yet another, bigger version of the 'high value engine', which was a good engine back in the 1980s in its 2.8 liter original, but every version since has had intake manifold problems, like most modern engines.
Alas the 3800 is totally discontinued.
I just thought I would set you straight on the whole muscle car bit. The Monte Carlo was actually introduced in 1970 to be Chevy's entry in the very profitable (at that time) personal luxury segment. The muscle cars were covered by the likes of the Nova, Camaro, Chevelle, and Corvette.
If your 3.5L Chevy V-6 is not blowing heat at idle, you are just probably low on coolant, and may have air trapped in your cooling system. I had this problem with my 2007 Monte Carlo, and I fixed it myself by slowly adding pre-mixed Dex-cool into the coolant over-flow until it was at least half-full. Do this with the engine fully warmed up.
After a week or a few hundred miles, check the level and add more if the tank is not at least 1/4 full. When I did this, I could actually see bubbles coming up into the coolant tank. This trapped air could possibly damage a gasket, if left too long, maybe even the head gasket, but you would have to run the engine low on coolant for several years before this would happen. Since doing this, I have had no problem with heat or coolant loss.