Here is my plan for the butts,

38 bang stick, not between the eyes just slightly behind them.

Depending on weather the current is ripping or not sometimes I would harpoon them first, using a harpoon with about 10 feet of rope attached and a float. Never cleat one off those big kid model butts can really pull. I have heard of them pulling the harpoon through the meat. Also if you buy a harpoon make sure the tip comes off easily. I had one the that the tip would stick on. It would go right through the fish and pull out with the harpoon. Then you would have to fight the fish back to the boat.

We fished them off the anchor, on Cook Inlet. The current really rips.

Also it would depend on how well it was hooked. Nothing like shooting a clients fish and having the hook pull put of the dead fish.(it only happened once)

Anyway I really like the bang stick, Halibut are great eating but any butt over 50 lbs can go ape [censored] once its in the boat. Most of my good fishing memories don't involve any broken rods, bruises or wrestling with halibut. I like to catch the and eat them not wrestle with them

As far as gaffing them, it depends on how high off the water you are. If you are close to the water I like a swing type gaff. Right behind the eyes.
JY