Should I spend the cash and get a scope mount and cylinder bore choke for it...
I don't want a scope on a shotgun... what choke tube is in your shotgun right now? MOD is often preferred for slugs. CYL bore is likely going to produce slightly larger groups. I suggest just buying some non-sabot slugs (15-50) and practice to get softball to 6 inch groups at worst and you will be fine.