Here is a pic of a Mission Style Bed. You will see that the posts on either side are at least 4" wider than the mattress. This is a King Size Bed. The post placemnt is so that the side rails will be a little outside of the mattress. In this case the side rails do not support the bed, they are just there to make the whole thing work.
dave take a bit of time and draw out your head board idea,, then like they have said yu need at least a 2 inch post or support piece to attache your rails too..using the matters size you need to allow a little on each side for the coverings but no more than a inch. between the mattress and the rails