First some simply arithmetic
You have 72" in height, less about 12" for pots, less another 12" for the light(s) and air space above them, less another 12" between the light and the canopy ... so 72-36 leaves you roughly 36" for your plants.
Now about (good quality) autos ... they're nice to grow but they're not the same as photos. If you haven't grown them before, they need to be started in the pot they'll finish in. Disrupting the roots in the first few weeks has a significant negative effect on the plants. Using something like Rapid Rooters in the center of the pot works well.
Because of the way the start out with more root development than visible plant, you really don't get to top most strains more than once.
Most reputable breeders will give you a height estimate for the strain and some will tell you how many per square meter to maximize yield. LST and scrog are good techniques for taller strains and SOG works well for smaller varieties.