66 Barracuda owners... Wheel question

What is the widest wheel/biggest tire you are running on your UNMODIFIED
rear wheel openings?I'm thinking about getting some adapters to go from the 4" SBP up to the 4.5" BBP wheels so I can go to a bigger/wider rear wheel/tire combination.
How much space will the adapter take up?
Would I be better off buying the Wheel Vintiques 4" SBP wheels that are wider?
I'm thinking I could go from the current 5.5 inch wide wheel up to the 7 inch wide wheel with no problem, but I wonder if the 8 inch wide would fit?
NOTE: I'm not racing, and I probably won't be driving the car more than 150 miles one way at any given time. Should adapters be ok? / safe?

THANKS!