i feel your pain, i just helped a friend change a thru-hull on a a 38 sport fisher that had a 3208 cat on top of where the transducer was. we also had to get the old one out. he had a flange wrench that was like a cresent with square jaws that worked fairly well.

Do the shoot thru hull ones not perform all that well on the screen type fishfinders?
The digital depthfinder I have has a shoot thru mounted in the bow, even with the thickness of the V it works. But I'm only using depth, not to show structure or fish.