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Most Impala wheels where six inches or less stock. There where some 15" x 7" wheels but they where usually on cop cars or on wagons. In 1970 I would suspect a 15" x 6" with an E78 bias ply as the stock tire if it has steel wheels, 15" x 7" wheels if they are Rally wheels. The 15" x 8" wheels where only used on the C3 and C4 Corvettes though they will bolt unjust fine on your Impala.
You also have to think ahead if you are planning on running disc brakes. All Corvette Rally wheels will clear disc brakes. The 15" x 7" Rally wheels off of C2 Corvettes as well as the 15" x 8" Rally wheels found on the later models as four wheel disc brakes where standard equipment from 1965 and up.
For 1970 B-body disc bakes were a rare option; and as such will be very expensive if you can find a complete set of parts to upgrade your car used. They were only used in 1969 and 1970. Because the engineers were not sure if the single caliper used on the X, F, and A body cars (compact, sport and intermediate models) they designed a super sized full floating single piston caliper that was also used on pick-up trucks.
You can use the 1968 Corvette four piston fixed bridge design as the parts will interchange but because of spindle differences you need an Impala front wheel hub that is not reproduced (though all of the other Corvette parts you would need beside the hub are reproduced). It will probably be cheaper to go with a COMPLETE disc brake conversion kit (calipers, rotors, brake lines , power assist booster and master cylinder with a new push rod to match for your brake pedal) from someone like CPP (another one of our sponsors). Or to bite the bullet and buy from Wilwood or Baer a big brake upgrade kit (using 13" or 14" inch rotors that will require larger than 15" inch wheels.
Standard steel wheels will not clear any disc brake combination unless it was off of a car known to be equipped with optional disc brakes such as a police car, and Rally wheels will only clear if they come off of a disc brake equipped car such as a Chevelle or Camaro that had single piston disc brakes starting in 1969.