You get single shot gun fairly early in game, and you can buy Caravan shotgun (holds two round instead of one!).
Slugshot is actually, a Slug instead of 7 pallet bunch together, in case if you are not sure.
Stick with Fastshot since you are shooting things up close anyways; 2nd Trait is purely personal preference.
As for Perk goes:
Rapid Reload (not sure if the .357 loading bug effect lever-action shotgun, but you would be fine otherwise).
HandLoader (recover more hulls, and give you more shot for crafting)
Stonewall (+5DT against melee attacks)
Cowboy (stretch lever-action shotgun's life spam, if it doesn't suffer .357 loading bug)