Yes, 315lbs isn't much, because that load wouldn't be on each post. The load would be spread out between the two.
Heavy duty J-Hooks attached to each post.
For the pull up bar, you might do a 2x6 on each side, connecting the two posts at the top. For portability sake, you could use two through bolts on each side. Then attach your pull up bar to one of the 2x6.