I want to make the two story 40 instead of 34. Is there anything I should know when adding onto the length.

Adding length is a piece of cake.   You extend the foundation, add more joists and floor, add more walls down the sides, and make it longer. No need to re-engineer things like joists and rafters that go across the width. Increasing the width would be a whole new kettle of fish.

As Don said -- same spacings as the rest.  No problem.

Check out Jimmy Cason's home, he extended it to 40 feet.  As you can see in the pictures, you just make it longer.