Will, MG Magic’s parts list shows the following,
Front bumper,rear engine kit 53” across
Front bumper front engine kit 57 1/2 ” across
Rear bumper MG Kit 55 1/2 across
Rearbumper MG Kit 61 1/2” across, special , no longer available.
Perhaps your kit was shipped with the shorter rear bumper because the longer one is not available any more.
If it works better to use the longer one on thr rear ,you could use a chrome bolt and washer to plug the crank hole.