There is a 140 macro lens made by Mamiya for close up work. I would try some test shots with this before you buy anything. The depth of field may or may not be good enough for you, but it will difinatly be sharper than a teleconverter or extention tube. It also comes very close to the 180mm for portrait work. You can get a used 140 macro in decent shape off of ebay or something for less than $600.
RB67: Teleconverters & Close-ups
in Medium Format
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