could easily build a Coast to Coast internal, and drain through the bottom of the tank (OF Tee shaped, think 2x4 on the flat sitting on a 4x4 post, or even the vertical box to one side, with your standpipe, assuming Herbie, in the 4x4 section).
RC has a calculator that gives weir length for flow. Not sure the reasoning but will give a water height of about 1/4" above the weir with the rated flow.