Vic - where are you located? I'll look around out here. You might also check ebay. Even if you don't seen any steering arms email folks that are parting out motors (midwest . . . freshwater motors are the best). All the steering arms on the V6's (not the inline 6) 135, 150, 175, 200 Mercury and Mariner from 1985/6 - 1998? are the same. I don't think they changed after that either, but you'll want to double check.
I recently changed out the entire swivel bracket and trim on a 1979 v200 motor to the newer style 3 ram trim and tilt. It not that bad a job if the bolts aren't locked up.
I have a mint old steering arm in my garage from that motor, but it's different than the newer ones. The Steering arms from the Single and Double ram Merc v6's won't work for your motta'
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Vic - Knock off the rust and dig into the cracks . . . you might not need to replace the arm. Those steering arm are solid steel and it takes a lot to rust one out.
. I was wondering if anybody else had dealt with this one before. Or maybe knows of a good surplus? 
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