Congrats! As an east coaster I was a grandpa and grandma (gramma) guy. Papa and Nana seem to be Midwest names that my kids use – (I personally find “papa” kinda weird). In my opinion “old man” is your dad, just as “old lady” is your wife. Our other grandma is “Oma” as she emigrated from Germany.
In the end, find what works, and be around and involved enough that the title has lasting and significant meaning for them.