Doesn't fit? Is this because you only need a 6 pin?
Well, if you feel that you're up to the job, then yes you could switch the plugs. Obviously you need to connect the wires in the same order. On the 6+2 plug, the extra 2 wires are only ground wires and not needed for a 6 pin (and obviously the plug isn't required either for a 6 pin), so it's optional if you even put the plug on.