Completely untrue. As a matter of fact, I can't think of any parameter of a k-member that would even come into question as a manual r&p is dimensionally the same as a power unit. I had a Griggs unit (cough, cough, currently for sale, cough) that would work with either. Any aftermarket K-member should.
I'd like to hear the reasoning behind whoever's opinion it was that told you that.