The picture of the pair of rifles, just goes to show, that Gibb's has good lines. Nice 8 bore!
I might even go with a .835" ball, Huvius, to allow a 12ox or 14ox. denim patch.
The thicker the patch, the more lube it carries. Good thinking on your part.
I'm sure a .710" to .715" ball (provided the bore is .720")on the Gibbs with a substantial patch should improve the 100yard accuracy quite a bit.
The 130gr. powder charge sounds about right for today's powder. I might even try 10 to 15gr. more.
A speed around 1,400fps to 1,500fps is a good goal for the Gibbs and 1,250fps to 1,300fps for the 8 bore.