The 3 bolt mounting pattern might be the same.... dunno, but the part #'s are different as are the part #'s for the radiators.
Here's what I do know about the fans, the 650 fan has a extra bracket hanging off of it that gives support to the big black plastic shroud that surrounds it. The 1000 doesn't. (*Might* be able to hacksaw it off.)
I have two 1000 fans for sale if your intersted.