The Moss Motors short hub kit is for TR2-3B, which have the short hub, and need one shorter still in order to mount the fan right way forward.
A stock TR2-3 hub will work in a TR4 with the fan facing the right way. Or, one can mount the fan facing backwards in any TR with no modifications to the hub. The blades are shaped and are probably more efficient when facing the right way around, but it still moves a lot more air than the stock fan, even when mounted backwards.
Andy --
10,000 posts and products available to tame TR overheating can't all be a hoax... All I've done to my TR3 is put in the yellow plastic fan -- backwards. On hot days, I watch the temp gauge creep up in slow traffic. Haven't boiled it yet, but it goes higher than I really want to see it. Fortunately, as soon as I get moving more than 20 MPH, it recovers. Also, fortunately, heavy traffic is fairly scarce and short duration around here. If I lived in LA or Houston, I think I'd have to take stronger measures.