To get rid of that much oil and not foul plugs, there MUST be a leak somewhere. I once had a '63 chevy with a 230 which used lots of oil. It turned out to be the rear main seal. It wasn't obvious - there wasn't oil on the bottom side of the car, and it didn't leave a puddle when parked. It just dripped on the road while I was driving. BTW, the seal leaked because the main bearings were loose. Also, check the oil pressure sending unit. They can get rid of a lot of oil when they leak.
