road:
Not sure about what you are describing, but here’s what I do in all cases.
The step board can be either a 2×6 pine board or a 1×6 oak board, the altter is recomended.
So the foot rest is 5.5 inches wide. Once installed you should have a small over hang or lip where the foot rest hangs over the base framing.
The lip should 1/2 inch and as much as 3/4″ depending upon your taste.
The plans call for the front OSB sheeting and paneling to be in place before placing the foot rest board. This is done to hide to bottom seam of the OSB sheeting and paneling.
In addition, some people like to dress up the base framing by adding 1/4″ veneer or finish plywood or even thin oak boards. In this case, go with a 3/4″ overhang and the venner layer will fill in 1/4 of that to produce a 1/2 inch over hang. Got all that?
If what you’d like to do requires changes in the base dimensions, you can go ahead and make those modifications, just be sure to test fit your parts before final assembly.