Sounds like you have it figured out. I'm not sure what your question is, but a photo might help trigger some ideas.

Can you post a picture?

if this bed has a traditional head, foot, rail configuration . You will need some type of Joining support for the rails at the foot. If the foot board is not used. This could be done with some basic lumber. The actuall amount of room that the "footboard" takes up however is typically not that much...at least in its added length to the bed. Visually it may have more presence if it extend up above the mattress.

If the current bed is in really good shape or even close to it. I would attempt to refinish it and use it as is. However if it is really something that does not fit your decor or in poor shape then simply remove the foot board and construct a new frame to fit the old ones place to hold up that end of the bed. If the legs are removable, you may want to remove them and install shorter ones. It would be a shame to destroy an old piece of furniture it if could be salvaged to its original shape.