I am in agreement with Elmariachi and others in that your photos depict a 427SO.
The block I did my build on is a NOS 68 SO block I found still in its cosmoline wrapping. The only difference I can spot between the two is the 427 marking in the lifter valley where some blocks have an X next to the marking and some don't.
I have heard and read a lot of different indications on what the X stamping actually indicates.
The lower of the two plugs shown on the upper right rear of your block is the housing for the pressure relief valve/spring mechanism. The darkened line running down from that plug appears to me to simply be thread sealant running down.
That's a cool find you have made, if you do decide to sell it I think you found an active market for it here.
