Shooter,
I have limited personal experience with your topic, but have gathered some info I hope helps. My first thought is R4 tires are not that good in snow. We discuss this often on the forum and I for one have never been convinced that R4 tires do that well in snow. My older 4310 would really move snow with the front blade, as long as it was pretty flat. Add a hill, or some icy snow underneath and it was skating, even in 4wd. The problem I see is that these machines are heavy, and will push alot of snow once moving. The R4 tires tread pattern seemingly does not "grip" that well as the bars are wide,flat and smooth. The tires are also hard, especially when loaded, and do not have enough sidewall give to increase contact patch when starting and stopping. Changing direction, or handling a heavy blade and blower will tax not only forward and reverse traction, but lateral as well. I have some areas here that my 595 garden tractor would manuver well on that I would not even attempt in snow to try with my 4310, 3720 or current 4520. I looked at some rear chains for my 3720, and I am no expert, but I really was concerned about chain rub in the fenders. I would be more concerned about the 43" Marathoners, unless you were to invert your wheels. Front chains are a "no-no" from JD. My neighbor damaged his front differential trying them , and although it was warrantied, it took some calling and begging on his part to do so. My opinion, for what it is worth, is to "do it right." You have a 3720 with a blade and a rear blower. There is no "tire" that will offer the traction of a set of chains. I would ask your dealer whether you could buy an extra set of rear wheels (sometimes they will have a set even at the dealer--from where someone changed out tires); buy them and do not load them (with the blower weight will not help that much) and mount chains. When winter comes, mount them inverted with the chains on. That way, you can switch back and forth easily and do it almost as fast, if not as fast, as you could struggle with a set of chains on tires that are barely able to fit within the fender wells. Yes it may cost a bit more, but you will have the advantage of chains without the potential damage. And over the life of the tractor, it will be a good cost to benefit ratio. My neighbor (same one), who until two months ago had a 4410, did this. In winter he swapped his usual wheels for a set with chains. He bought them off ebay and could swap wheels out in less than 20 minutes with a jack. His cost was about $400.00 total and he got much good use from them. Ultimately, when he wore the tread out on his original tires he just swapped his newer winter wheel-tire combo and removed the chains for year round use.
John M