How do I mount a 70" TV to wall when the studs are in the wrong place?
Most every mount I've seen has several holes through which it can be secured to studs. This is to account for different stud spacing, as well as allow for several lateral positions so that the TV is not off center.

Double check your mount instructions, to ensure it will not accommodate your stud spacing. I'm very suspicious that the mount says it must be mounted between studs, as you want that kind of load directly into at least one stud. 

If it won't, then attaching 3/4" plywood to the studs, and the mount to the plywood, is indeed the way to go.