gmc crossmembers were completely different than chevies. The 6 and 8 cylinders used the same crossmember. there was an extra plate riveted to the v-8 crossmember for an engine mount. I believe the gmc 8 mounted under the balancer to the crossmember. you can swap crossmembers by drilling out the rivets. Even with the gmc crossmember, it should be a pretty straight forward swap. blue chip crossmembers are the same for 55-57 and 58-59. the 58 and 59 are eazier to swap because the radiator and front sheet metal do not mount to the crossmember like they did in 55-57 models.