The end of the patchbox cover will need to be rasped to follow the contour of the buttplate. Once you've got it to the length you like, and the end of the cover fits the flow of the buttplate, NOW is the time to fit a plate to the end. You'll end up taking a little more wood off for the approximate thickness of the brass end cover as you file the brass to the contour of the buttplate. It's nice to have a little extra brass for filing.
If the cover ends up being just a touch too long with the brass piece already installed, you can shave a little off the front end for the final fit. Then fit your catch spring piece.