Although I've converted standard top oiler blocks to side oiler design by coming in through the main caps and feeding the
oil at the bottom of the bearing, the block ideally already needs to have the casting in place to accommodate the side oiler drilling.
During the eighties and early nineties, I came across a number of industrial center or top oiler 427's that were cast as sideoilers but drilled only as top oilers. On those blocks, I never felt that the side oiler design was worth the additional expense or trouble to convert them to a sideoiler design if the appropriate
oil gallery mods were performed on the top oilers.
Personally it's not worth the trouble on a used block even if the casting provisons are there for the sideoiler.