The 525SC intakes are very hard to come by used and Merc wants about 4 times the $$$ that they are worth. Mine had salt usage before I got it and was flushed and well taken care of. Still it had some heavy salt damage around the water ports and Tstat. I think it cost me about $300 to get it all welded up. I think the studs are stainless, but not sure. Chances are its seized because of salt and being dissimilar metals.

would this work , the only diff i see is the thermostat location is slightly diff but i have a holley strip dominator on there now with the same location, i dont know maybe the sc will interfere with the 4 port housing

http://www.holley.com/6130WIN.asp

Put some pics up of the corrosion. If that is ok, I'm sure we can get the stud out one way or another. For much less than $550.