-6 will push them out even further and i can almost guarantee rubbing and most likely pretty bad. (-) offset results in the wheels "farther out," and the opposite for (+). generally, it depends on your vehicle to classify farther out, flush etc. offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel, think about it as the distance from where your rim sits on the studs. 0 offset means there is the same rim width on either side of where the face of your studs sit.
just think about it logically. 1.5" wider rim + 0 or -6 offset=much farther "out"= bad rub=not worth it IMO. with the addition of adapters that are sometimes sketchy, i wouldn't even think about it. i had some pretty - offset rims for my taco and 265's flung anything and everything on the side of the vehicle. a negative offset isn't going to solve your problems, yes they will be farther away from the inside (uca etc.) but think about turning with an extra 1.5" of rim. fender rub is going to be nuts.
its understandable that you want to keep your tires, but just consider shopping around for an 18", 7.5" or 8" wide rim in 6x139.7 (5.5) at 0 offset and you will be much happier and then you can sell your spiders and offset the cost by $100.
as it stands now, with the rim you want being wider the tire is going to be "stretched" and that will not make the diff happy when you throw your old spare on that isn't as stretched when you get a flat. thus meaning you will have to buy 5 rims+adapters and do a CONSIDERABLE amount of trimming.
i would take blue4run's advice and get them powdercoated bronze or something. the color is up to you. [personal rant- murdered out is and has been played out.]