Rust in the two tanks is so common, Ford/DeTomaso nickel or chrome plated them inside and outside on L-model Panteras. Remove the black paint to see if yours is plated. POR-15 may delay the inevitable. The big water pipes are mild steel (exhaust pipe) and commonly rust through near the ends. The fix is a good grade of aftermarket stainless steel from the vendors for tanks and water pipes. Paint if you're into the stock look.
A peculiarity of Ford-era Panteras is, one undercar pipe is two piece while the other identical pipe is one piece. The long pipe cannot be removed except with the entire powertrain removed first, or by cutting it in two to match the short assembly. I wouldn't waste time nor effort with refurbishing stock water pipes. I also recommend discarding all stock multi-piece Italian hose clamps and Pirelli rubber hoses before nature does it for you on the road....