Is this possible? What should I look for to prove this car is an SS?

all super sports 69 and later had discs from the factory. No ifs ands or buts.

Disc brakes were part of the SS option package as mentioned. Build sheet, Protect-O-Plate, or some other written documentation is the only PROOF for 69-72 Chevelles except the 72 when the engine size was incorporated into the VIN. The SS option was MANDATORY for the 72 454.