Is my car running too cool?

Mine is basically the same, but with an electric fan. Tstat is 180* and stock replacement radiator. Recently, while driving over the sierra's in a snow storm on the way to Reno, it wouldn't get hot enough for the heater to be effective. Living in CA, I can live with it because it's normally fairly warm in the winter, with hot summers.

I would suggest going back to the stock 195* tstat (190*-195* is a debate in itself, so I won't go there). The tstat is designed into the system to maintain the engine temp, so it shouldn't open before that temp is reached, and it won't close until cooler. But in very cold weather the radiator can cool to the point that the tstat rarely opens, or the engine just never gets hot enough to open the stat. The stock tstat should not be a problem in summer if the cooling system is in good shape, with everything operating normally.

The problem could be the flex fan. They pull alot of air across the rad. Unlike the the stock fan clutches design, that work off of temp to speed it up and slow it down.

I would agree with that pro50sc if in stop and go traffic, but at normal speeds the flex fan blades are flat and don't blow much air at all. The stock 5.0 clutch fan is one of the best ever designed, and if in proper condition, will flow with of the best electrics and blow away flex fans. Most of us go to electric or flex fans as a weight saving effort, as the stocker weighs in the neighborhood of 12lbs.

Thermostat stuck open?

You're moving plenty of air with the flex fan, but if you want to swap back to the stock fan do a google search on "Performance Parts, Inc.". They sell a "special service" fan clutch that came standard on the police-package 5.0's. Sorry, not familiar with the price. They are supposedly alot more durable than the stock replacement. I would try going back to the stock tstat first.