The youngest of 105 cars in the US is 52 years old now. A lot can have happened. The reserve happened, a GTA Jr for sale at Rapley some years ago had steel front fenders according to the seller and a former owner. Probably an expedient race-car repair with parts that were at hand?
Get the Dasse Arese book to see period pics of the assembly line, and the GT book which covers GTAs also. I have trouble believing there'd be a reason, or easy method, to graft alum fenders onto an otherwise steel GTV. Maybe over at Autodelta, etc, but on the production line? I realize they made GTAs there too.