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Dont know about the distance, but the main difference between a big block shroud and a small block is the opening for the fan.
If I remember, the small block opening measures 19inches, where as, the big block measures 21inches. Most late model 350 engines come with a fan that is 19 to 19.5inches, so its too big to fit in the standard small block shrouds. ONe trick, is to use a fan from say a 1989 S-10 that had a 4.3L V6....that fan only measures 18.5" and will fit, although clearance is a bit tight.
If you go to the dealer and ask for more info, there are different part numbers for the thermal clutches. The 350 has a standard one, or a heavy duty one, and I think even another if the rear end ratio is a 3.73. The 4.3L s-10 fan will bolt right up to any of the 350 clutches. But if you want to use a 350 fan/clutch, you will need to get a big block shroud....or if you can find one, a shroud that came from a gmc with a 305 V6 works perfect. This is what i went with.
Either way. good luck!
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