Will a deep 727 be be the same as a stock 518?

Not typically, the typical 518 pan is about 1" deeper than a stock 727 pan. Most deep aluminum 727 pans are 2"-3" deeper. This will vary be design, I do think the Mopar deep Chrome 727 pans are close to 518 depth though.

The valve body on a 518 hangs down in the pan more than a 727. However, because of the pickup and dipstick design, as long as the pan doesn't hit the filter they can be interchanged.