From what I know Yes you can put it on the wrong way. There is some other things to consider as well. Such as if you have a 235 with a short shaft water pump. If you have a short shaft then the you may have bent the blade a bit and its now hitting. Simply bend it back. Or as you said "its on backwards" Post some pics or look at some of your old photos and compair. Good luck.
If this is your engine in the link below you have a short shaft water pump. A lot of the time when folks install this pump the fan hits and requires bending of the blades. It dose not harm the fan just simpley bend it out. I have dne this as well as others.
http://i1130.photobucket.com/albums/m533/peterharbo/IMG_7097.jpg
