was this originally a v-8 truck or a 6 cylinder. the rear crossmember should be the same for 6 and 8 configurations unless the truck was a hydramatic equiped truck. If I recall on the 57 gmc the radiator and front clip mount to the front crossmember. for a factory v 8 the crossmember was the same but there was an extra plate riveted on for the v 8 mount. If you are using the stock gmc 6 front motor mount... they are really hard to find. Nobody reproduces them to my knowledge. you may be able swap a chevy timing plate on the engine and mount it to the crossmember with a chevy motor mount.