FWIW,
I was told by a guy that by the late sixties, the 427 block tooling and molds were worn out, and there was core shifting and thin skirts with some of the castings. Ford knew they were D/C'ing production, and were in the process in making the 429 the new HiPo block. Rather than get all new tooling to continue production of 427's, which were deemed inferior to the new 429 engine, they made modifications to the old molds, incorporating ribs in the skirts to strengthen them up, as thin skirts was one of the problems of later 427 castings.
Not all replacement blocks had or needed the ribs. I believe the GTE cougars in '68 came with 427 SO's, with hydraulic lifters, and no ribs as with some of over the counter replacement 427 SO's. You can take a hydraulic block, tap and block the lifter
oil galleries, and/or run shell lifters, and run a mech cam. Obviously, if you don't have the hydraulic lifter galleries, you can't run a hydraulic cam.
I also heard that when Ford was machining a 427 SO block, and cut too deep when machining the driver side engine mount pad, into the SO
oil gallery, they then finished drilling the block out as a top oiler, and used it as a marine engine.