I think that they're 7"...
Give'em a proper cleaning before you put them on. I use a good polish, followed by a good wax. That'll keep the dust off'em and make cleaning easier.
I wonder if they need hub-centric rings? You might check that. And also if the lugs are included. Different wheels require different style of lugs...
And as to offset...just watch your tires for rubbing and listen for it too...
