i just used the gas door and unfolded the lip, on the 4x4 box the gas door has a big curve in it so i just thought it would be easier, look fine to me, nice and clean
If I were you I would make sure the corners are a slow radius. If they are two quick or come to a piont when you weld the plate in it will warp an unessesary amount. Best way is if yu can cut the rolled edge of the bed but the peices together and lightly hammer the weld as yu go to bring up the weld as it shrinks. makes for less fillerwork.