I had the same dilemma for my '31 S/W sedan. It is trickier, because the seat frame is all-steel. I decided to cut slots for the belts to pass through. I also lined the edges of the slots with folded pieces of thinner sheet metal, so the edges could not damage the seat belt webbing. This photo shows what I did:
http://www.dougbraun.com/model_a_gal...g2_itemId=5884
Doug