The radiator cushions with locating pics are usually chev style without shroud. Not sure what you are pushing them into on a 400, or 403? The repros are a combination of both styles and need trimming off rather than produce 2 types.
Http://www.firemanjax.com/wqik.net/1978 trans am t top/1978 Trans AM T-top Gold engine compartment passenger side view.jpg