Not sure about the diesels, but the gassers have such a big bypass that they struggle to warm up to run without really working them. I cover about 2/3 of the radiator on my M during the winter. It is hard on the fan blades, but it always warms up to run and I don't have tons of cold air blowing on me (it's my loader tractor and used daily year around). I've always been of the mind that it needs to get to operating temp also.
I've wondered about doing a little welding in the bypass area to reduce the size and help them warm up. I haven't wondered it enough to take time to do it, though. There are always other projects first and it works just fine like it is.