LaJolla,
I don't realy know AB Miller well, but I like Miller Draft A Lot
. Anyway, I think they do not come with any gun breakaway system. I have now a "48 Special" , looks like Mahagony wood, a friend of mine wanted me to help him service the trigger and line release mech.
You first need a gun-breakaway system which if you see on a Riffe, it is a small hole on the gun teak stock near butt area. The idea is to insert the breakaway bungie into it so that when a fish is big enough it will be able to pull it out and the float will take charge. The shooting line termination connection where u are now attaching to the muzzle will go to this gun-breakaway assy.
If u want to save money and do not want a dedicated shaft for float use, you can make an extension line from Kevlar, so that u may use ur current set up also with float line. In order to do this, there must be a shivel at the muzzle so that changing shooting line set up is only by unclicking the shivel.
First u must get or make some sort of a reliable gun breakaway for ur AB Miller, it can be a hole like a Riffe or and additional L shaped piece of stainless with a hole or whatever genius design people been trying these days. The main idea is that you must be able to swim into current and make fast movement and the breakaway should not release itself by these drag. It should only release itself if the pull of the fish is strong enough...perhaps 6-8 pounds of pull. If you use bungie or any resistance design, u must try it wet not dry.
Do some research on the net and it will give you some idea, if u grasp the basic idea, a few in here can teach you how to make one.
Have Fun,
IYA