427's all had cross bolted mains so you should find 3 bolt heads running along (just above) the base pan rail. I believe some of the early 406's also had cross bolted mains.

The ribs are supposed to be there for the later blocks, they were meant to add strength that early blocks lacked. A great site for FE info is fordfe.com, a lot of knowledge there. FE's can be terrible to identify. The surest way is to measure the bore & stroke.