I find they do straighten , if you soak them long enough, it is normally because they have been cut from wood close to the heart wood
i soak all parts then glue the top and bottom rails to the longer sides while fixing with a vertical thin long nail down through the tenons ( ie not thru the side face of rail ....you then have a wobbly box, i fix the plastic runner to the top of the shorted sides ( i use new metal one) then tap in the shorted sides having put glue into the channels) until the runner is a frame top depth below sides/top rail, ie a frame top bar should be level or 1mm below with the top of the sides when resting on the runners...no need to measure as required in Th**nes instructions and a variaint of the maisemore instructions download
I square up the box on a crown board and then nail through the back of the top and bottom rails into the shorter sides.( if the super sides were warped and it needs heavier fixing i the nail though the sides and through the tenons)