if your dead set on using this gun, you csan have the barrel sent off to a GOOD gun smith, and if there is enough meat on it, they can cut barrel back a little and tread it fo chokes will fit in it(screw in chokes)
you will loose any collectable value to the gun, but it is an option.

but then again the price to buy another newer shotgun might be cheaper
a decent used modern pump shotgun should be had for about $ 150.00 bucks or even less if you shop about?
and will be way more accurate than this full choke, that might toss slugs at random points of aim??
which can lead to a missed deer or worse a wounded one?

That's something that will be individual to your gun. They don't cost that much. If they still sell them in boxes of 5, buy a box of each brand, and try them.

If you are set on using this shotgun I would say grab a 3 boxes each of 2-3/4" Remington Sluggers, 2-/4" Brenneke KOs and probably Federal Vital-Shok and hit the range and see what you can do with it. I would shoot atleast a box or 2 boxes of each brand with some time between each brand to let the barrel cool. I don't know how well you can absorb slug recoil you you might want to consider a shooting partner to share a shoulder.

Another thing. You say this is a one time hunt so I would think you want to make it count when the moment comes. If you don't have some accuracy or the range you are looking for maybe you can borrow a slug gun from a buddy?