Not sure on the exact depth (5 - 5 1/2") but I'd add the stock style pickup is supposed to touch the bottom of the pan. The pickup has a shroud that rests on the pan bottom to hold the pickup screen off the bottom a little to allow flow. It sort of a failsafe design so the system still works if the piclup moves around or the pan bottom gets bent.

I used Mancini's pan/pickup kit and it worked out fine.