The bumper reinforcement is bolted to the radiator support. The USDM radiator support needs to be swapped with the JDM one. But you can take the USDM reinforcement and bolt it to the JDM radiator support. Minor trimming of the JDM bumper and it will fit over the USDM reinforcement.
There is also a way to cut away the top of the USDM radiator support, but I always thought that was kind of half-assed. The hardest part of the front end swap is drilling the welds correctly, breaking it free without damaging things, then aligning the new one correctly to weld it in. If you mess up any of this, the parts may not line up correctly.