DML FAQ : Engine : What thermostat do I need?
All Robert Shaw part numbers
330-180: I have seen a couple of posts from people who have said they installed this t-stat in their 4.7 V8s. Perhaps this is the origin of this P/N. I can't confirm wether or not this is the proper t-stat for the 4.7, but it definitely is not listed as the proper t-stat for the 3.9/5.2/5.9.
370-180: This part number has been thrown around a lot, but the only Daks this is applicable to are those with 5.2 or 5.9 V8s which have been equipped with the Mopar Performance MPI intake manifold.
400-180: This is the part number which is listed as the proper t-stat for the 3.9/5.2/5.9.
GT755-180: This is the "generation two" version of the 400-180. The "gen 2" is the "spring and plunger" style, like the stock t-stat, whereas the 400-180 has a kind of cylindrical base, and it opens via a rotating action, I believe.
The Gen 2 is the more expensive t-stat, and comes with a limited lifetime warranty. Either will work, its just personal preference. I don't know enough about them to discuss their relative merits, unfortunately. I use the standard (non gen-2) style, due somewhat to price, but mostly just because of the good things I've heard about that particular t-stat. I have heard they are less likely to clog or stick than the plunger and spring style, so that's what I used. Based on the price and warranty from Autozone, the gen 2 is the "better" t-stat, but I'm happy with the original design so far, so if it ain't broke, I'm not going to fix it.
Note: The "180" in the above part numbers is the heat range, and referrs to the degrees F at which the t-stat will open (approximately). "195" is the stock range; there is also a "160" available for most (if not all) of the above part numbers.
The above info was gleaned from personal experience and a painful search across many different model years at Autozone's web site.