What is this and why would it be there?

Johnny Mac said
Look at the machining on the opposite bank. Think it had a roller cam at one point...And has bad core shift, and that JB goo is covering where they punched water jackets on the other side??

Our virgin 340 block had exactly those same machined grooves in the sides of the block above the lifter bores. So I do not think that is a sign of a roller cam being in there before. (Our block had sat in storage for 30-40 years with Dick Titsworth in Toledo. It might have been some sort of block for his '71 Cuda ... did not ask!) You can see the lower edges of some of them in this pix; they were above each and every lifter bore.