Yeah...Whip has it right...also, it's not so much the draw weight, as it is your draw length. In other words, your 40# is nothing unique to a 60" longbow, but if you had a 32" draw length, you'd likely want to consider going longer with your overall bow length. It's a comfort thing more than anything but it is also a function of design and intended use i.e. you do not want to draw the limbs passed their intended draw range for fear of breakage, overstress, etc. Especially true when considering selfbows.
Kris