This is for the five people in the world with this exact problem …

First, you have an antique three-forths bed and you want to use a standard full sized mattress.

The solution is to create an insert that replaces the box-spring and extends over the side bed rails.

A few years later you need the bed to be a little lower.

The solution is to create a new bed frame with its own legs and inset ends so the antique head board and foot board have something to attach to.

bed frame to reuse antique head and foot board
bed frame to reuse antique head and foot board

Yeah, like I said, this is for the five people in a world of 7,476,562,882 people who have this problem.