I seem to remember that during the Stalybridge blockade, and North TPE services were diverted via the Calder Valley, the paths of the Northern Manchester Victoria-Leeds and vv semi-fasts were taken by TPE running double-set 185s between Manchester Airport, Piccadilly, Victoria & Milner Royd Junction and vv.
Those services called at Rochdale, Todmorden and Hebden Bridge to compensate for the loss of the Northern trains, but passengers for Todmorden and Hebden Bridge could use only the front 3-car set.
Therefore, presumably, Todmorden and Hebden Bridge would be two of the Calder Valley stations which would require platform lengthening, and if they were to stop at Sowerby Bridge I assume they would also need lengthening, particularly the present down platform, which is now much shorter than it used to be.