You're leaving out some information about your Slug gun. Are you running a Saddleback Mount? A custom drilled/tapped receiver? You keep talking about Weaver Mounts, but you need a "Base" first to even put the Weaver Scope Mounts on. What have you got?
Yes, you'll need some serious eye relief. I would suggest a good Scout Scope that will give you the scope eye relief you need. A good Red Dot in Leupold, Primary Arms, Bushnell Trophy to name a few. Stay away form those that sell under $100. You might even knock out a $250 Red Dot---who knows. You need something that will handle extreme shock. Go to SWFA and take a peek at what they have in good quality scopes. Give them a call and ask questions about the scope you select for the job you want it to do.