I may be thinking of something else but I could of sworn I read somewhere that the poly 318's had really thick cylinder walls and could easily be punched out to a 4.040" bore to make it a 340.
Someone correct me if i'm wrong. I don't want to put bad info out there for more confusion.
Now you are talking about a poly 318? They haven't made those since 1966, then in 1967 they started making the 273's, then 318's and 340's. The polys were a completely different block than the later s/b v-8's.