The poor mans Contax 645 in terms of what? Image quality? Not saying the Mamiya system is bad, because I have one and love it, but the Contax does have Zeiss lenses like your Hassy..
So, about 645. So your Hassy take 12 6cm x 6cm shots on a roll of 120. A 645 system takes 15 or 16 6cm x 4.5cm shots on a roll of 120.
This means each image is narrower on one side so you can fit more shots per roll. The image would look the same as if you cropped 1.5cm from your 6x6 shots on your Hassy. Of course you will only shoot 12 shots per roll as apposed to 15/16.