Foiling the 68 GTO bumper would work if they were the same shape, but you could develop your bodyworking skills converting one and then foiling it. The grilles would be tricky for the Tempest, but the open light GTO grilles shouldn't be too hard.
I have one of those old MPC 1/25 68 GTO's, it's a survivor from my youth. It's had the dip in the purple pond and will be rebuilt as the "Woodward Ave." add car thanks to some Modelhaus resin parts. ( I couldn't build them stock back then)