Cool find. Their SS/IA car may or may not be a different car than the Stock Eliminator car. Those classes were more similar to each other in that era than they are now, so it would be a good idea to get rulebooks from that era and try to "date" modifications you find on the car that would move the car "up" to SS from Stock.
Most of the Stock/SS cars were picked up from dealerships, or in some cases bought off the street; a BIW would be a pretty special effort for those classes at that time. I'd start looking at the body very carefully for weight movement, omitted items, and clever modifications. Look for aluminum fasteners, check the wheelbase & wheel location, engine location, and the floor, especially at the rear. You may find some lead-filled tail lamps like my car, or a rear bumper that's way heavier than the front.
Keep us posted on what you find!
