"A long board air sander is good option but it seems to me that
has much longer tools especially for two people.
Do you thing it is possilbe to get any 70" long board?"
You can check out a marine supply shop for the larger sizes, 3M does make much larger sizes.
for 70" you could laminate 2,3,4 layers of 1/8" particle board and fit handles as you need.
Even on large boats the curves keep changing, 70" would be completely impractical for finish work. Much easier to make matching blocks for each area from hard Styrofoam foundation insulation boards from a building supply store.