I would try the modified choke first. I had an old SKB/Ithaca 900 modified choke that shot slugs like a rifle. Just enough squeeze to steady the slug.
Another thought...if you will be using this gun in the future as well, Mossberg 500 barrels are a direct swap with your 88 and you could either get an iron-sighted "slug" barrel or one with a cantilever for scope mounting.
@LiveFire also makes a very good point about trying various slugs. Not all slugs are created equal! My slug gun is a smooth bore (rather than today's rifled slug barrels) and is deadly with Activ deer slugs. Not made anymore, but I was lucky enough to lay in a case of them.