When I built my 400s, best practice was to run the AC up and over the fuse holders, across the top, then down to the thermo breaker, keeping that AC away from the boards.
Evidently Joe gets lower noise numbers taking it straight across. Very similar to the AC leg on the 700s.
Joe, do you keep the run tight against the backplane board, or pull it out equidistant between the backplane and control board?