I got three Pioneer 10's('98 models) into an old '93 regular cab Ranger with a sealed wood box. The internal was 3.5 cu. ft.
The guy wanted something in his truck, but didn't want to sit into the dash with the seat all the way up. I laughed!
The 8" will be more shallow then the 10". Putting subs into a single cab MINI requires room being used. You could do 1 8" sub or 4 8" subs and the seat position will be the same. Like wise with any sub.(unless it is a 60/40 seat and behind the passenge seat gets the sub)
4 8", 3 10", or 3 12"(a shallow one) will fit with some leg room.