Do not shoot the modern H&H Paradox loads through that gun. The H&H Paradox guns were and are made overbore. The bullets in those loads are ~.735" diameter. I would start with bullets sized a 2-3 thousandths smaller than the bore.

You say the rifling is polygonal. You should check to see if your gun has the oval bores where the narrow portions of the oval are significantly tighter than the wider portions. There was a little discussion of those here: http://forums.nitroexpress.com/showflat.php?Cat=0&Number=187622&an=0&page=0