What I think would work is to make a wooden buck out of 3/4 inch plywood of the radius you need and hammer the sheet metal over the wood buck. the wood needs to be very accurate in shape and radius would need to be sanded smooth, this is timely and alot of work but ifstart with a large hammer and finish with a small / flat sheet metal hammer you will be surprised at the results. It could be finished in metal with enough work or you could body work it .