In previous threads discussing your "261", the casting code indicated it was a 1960-62 block, most likely a service replacement as there was no stamped serial number.
A check of the
GM Heritage site for 261 engine specifications for all model years 1960-62 indicates they were all originally equipped with "mechanical lifters" (solid lifters, not hydraulic).
Although they were originally equipped with solid lifters, I can't say if the blocks were or weren't drilled to allow for the use of hydraulic lifters???
Perhaps someone with greater knowledge and experience with the later 261's can come along and help answer your question?
I would hold off on the lifter order/purchase until more info can be provided.
Dan
