Re: 1985 7.5HP Mercury swivel tube
The 85 7.5 had a plastic coated swivel tube. Older models maybe '70 minus had carbon steel uncoated tubes that were a lot more apt to rust and freeze up.
to pull the swivel tube you need to pull the power head off and then the bottom cowl. the tube is secured in place at the top with 3 bolts that hold the tiller arm bracket in place. you can see these bolts from the side just under the bottom cowl. After all of this is removed the swivel tube slides out from the top.
I would try lots of liquid wrench and PB blaster plus pump grease before I would take it apart.
if and after you take it all apart, if the swivel tube is hard to pull out from the top, try a slide hammer. you can hacksaw it out too as a really last resort.
Good luck, these are great little motors!