The front crossmember is the big difference. On the 28/29 chassis, the pads for mounting the radiator are raised, whereas on the 30/31 chassis, the pads are level with the portion of the crossmember that slides under the upper chassis rail. The 30/31 radiator is taller than the 28/29 radiator, so mounting a 30/31 radiator on a 28/29 front crossmember will lift the radiator and radiator shell too high to get proper hood alignment.
The center crossmember on the 28/29 also does not have mounting holes for mounting and using the 30/31 stoplight (brake light) switch. This is not as big an issue as you can still use the 28/29 stoplight switch which mounts off the transmission tower.