The best spot is right at the back of the gill plate, just below the pec fin. This is a small spot, just in front of the collar. If you miss and hit the collar or skull you will watch it head for the bottom and have to reel it up again! The reason I like this spot is that it hits the major arteries near the heart. The fish will bleed out quickly and the struggle will be over in just a minute or 2. Having the harpoon further forward also helps you control the fish. I run an A2 ball on my line, but have yet to deploy it. It's backup. My practice is to hit the fish, drop the harpoon handle, grab the rope with both hands, wearing gloves, brace the line over the rail to my thigh, and hang on. The fish will thrash like crazy, hit the boat, and water will fly all over. Don't hold her hard, make it a tug-a-war, the secret is to keep her nose above water. With her nose out she can't pull all that hard, just thrash. IF she gets her nose under watch out! She will have full traction, you can't hold her, dump the rope out of your hands and let her take the bouy. I have have harpooned and landed over 1000 large halibut in Alaska, including two 84", 330# fish. NEVER-NEVER-NEVER tie the rope to the boat!!!!! You can get away with it on smaller fish, under 100# or so, but the big ones break stuff when they hit the end. Another charter captain up there from Oregon didn't believe me and broke the wire leader on 5 brand-new heads before he went to a ball. Note, the guts are a fool-proof target as long as you penetrate both sides. It's harder to control the fish with the line farther back. The steel leader to the head will cut up the side you hit her on as she fights, but the back side next to the head will hold if you don't tie it to the boat.
76" here