If you have a good heat gun and several good clamps and some hardwood blocks you can clamp on three sides of the bulge then heat the bulge and either tap it down with a flat faced hammer or clamp it flat and let it cool. The hammer will shrink it better but either way it may take a few treatments.
The heat will make it bulge more and hammering it or clamping it flat will force it to shrink, if you use the hammer tap it gently, either way you need enough heat to make it bulge and it needs to cool so you can see how it reacted and how far you have left to go.
Finally got the chance to rough in the transom wood for Lisa's boat. The board of mahogany I got was *just big enough to replace the old boards. However, the original outer boards were short of the bottom by about 2". To add those two inches and the two inches to raise the transom height, I'm short on mahogany. Plus the cabinet shop didn't do a great job of planing the board on the ends.
There are a 100 different ways to skin a cat, and there are no easy solutions sometimes. As John said, Taking it to a machine shop is the easiest, and less of a headache for you, having the proper tools for this is a huge help.
The best advice is i can give is... patience. I love Kano Kroil Penetrating Oil. HEAT OIL, HEAT OIL,,MORE OIL Don't forget patience. Over a few days of doing this.
If you do decide to cut the head off and drill into, use cutting oil so you don't burn up the bits, I would NOT be tempted to use an "easy-out" ether,Those things break and the small ones break easy. if you break an easy-out in the bolt now you have to deal with a chunk of hardened steel as well.
Finally got me a map gas torch today and went to town on the '57 Evinrude. Well, towards town.
I got one of the screws out, popped another head off, and the last two are starting to... cam(?) out. What's the next step towards getting these out (without a welder)?
in this situation, I would use a carbide ball end cutter on my air grinder to get a good center point then I would drill it out
with a small drill bit, through the bolt, then I would get some penetrating oil in the hole to the bottom of the bolt
use a larger drill to get rid of the head so I can disassemble the unit,then go back and drill out with a left handed drill bit and hopefully the heat of drilling and the penetrating oil will let the bolt unscrew itself !
have you tried to tighten the screw first after giving it a good sharp whack with a hammer? some times that is all it takes to get them free
I just did that on a 70hp Chrysler and a 65 merc I am working on