that's what's in the PDF of the mega2560 schematic! damn it this sort of thing pisses me off: the reference design that arduino.cc released (the actual eagle files) are hopelessly out-of-date.
When and where did you get your copy? The versions I see now at https://store.arduino.cc/usa/arduino-mega-2560-rev3 look updated and current.
uncertain about what (low-er cost) single-channel op-amp to put in its place?
LMV321 ? Seems to be available from several vendors, and on ali-express for cheap. Beware the non-V version, which isn't a rail-to-rail-y.