Silicone tubing won't shrink much, however there are a near infinite number of sizes and adapters and clamps. Sketch out what you need and then hit up the turbo modder stuff on eBay. That's what I did to adapt the tractor radiator into this bus, as it had IIRC a 2.5" bottom outlet and the Cadi water pump is 1 1/2" or so. In my case, I used the stock DeVille lower hose, and simply put the radiator end onto the coolant pump. Had to cut it and install a pipe segment in order to get the 90 degree twist to the right angle. All the contortions actually wound around the oil pump like a factory fit, it looks perfect! Then I used an exhaust pipe size adapter segment along with the silicone hoses for the rest of the size reduction. So everything clears, has lots of room, and allows plenty of flex for movement of engine and frame/radiator.
I think I spent about $45 in total, including stainless steel clamp bands, so it wasn't super expensive. Well, some of the steel clamp bands turned out to not have stainless steel threaded portions :-( But they do work and clamp nice and tight. I added a few pics from back then, to illustrate.
