Had a 58 Green Mountain for about 10 years. Had a few 54s too in T/C / Pedersoli barrels. I could always workup a beefier load using the 58 and shoot it just as accurately as the 54s.

58 would be my suggestion for use. A 280 gr. roundball will do the job..... just avoid head-on's with shoulders. Try to obtain a work-up load using a wad behind the patch. You don't want any wasted ignition. You want every ounce of energy behind that ball.

If the wad doesn't help / maintain accuracy, discard the trial range-work using it.