What a beautiful rifle, buckstix. Great score.
Jim of course, is spot-on with his recommendations. All good advice, imho.
I fully agree with Neetsfoot oil or Trackofthewolf's mink oil for lube. I've never tried the liquid olive oil myself. These big bores use a lot of powder, which produces a lot of fouling, 56% of which is particulate fouling. Much of it remains in the bore, so enough lube (thick enough patch) is needed to keep it soft to allow the next load being pushed down the bore, as well as being left in the bore to allow the next shot being loaded.
Jim's advice on 10 ounce duck is good. I use 10 ounce denim which I currently measure at .021", compressed in calipers as hard as I can squeeze between finger and thumb on the tines.
Patches need to be wet.
I see the muzzle needs a quick 'fix' to allow loading snug loads. Here is a copy of a post I made recently on another forum.
"After being in this "game" for many years and having a penchant for extracting the best accuracy possible from a given barrel, there are a few things I have found through experience to be vital to have results that I can live with - and quickly.
The first thing is to smooth the machined crown of the muzzle. Machined crowns usually have about a 45 degree angled cut from outside the groove diameter to the bore (tops of the lands).
This 'crown' leaves 2 short corners that the patch and ball must "conform" to before they are fully entered into the bore.
The method I use is either with a lathe or just the end of my thumb pressed into the bore with either 320 emery cloth or 320 wet/dry paper between my thumb and the muzzle's machine-cut crown.
With a rotating motion of the wrist, those sharp machine cut-edges are smoothed. I turn the barrel or gun 90 degrees every 15 seconds or so, so that the "smoothing" is uniformly progressed around the muzzle. What the end result becomes, is a finish at the crown that allows a VERY snug ball and patch to form into the bore, giving a seal on both the bore (tops of the lands) and in the bottom of the grooves.
Once this is done, the actual bore and groove dia. are measured, then a ball (mould) of .005" under bore size is selected, along with 10 ounce (.021" to .022") denim is selected. With .32 cal. I start at 25gr., 30gr. with .36, 50gr. with .40, 60gr. with .45(all 3F, then 80gr. 2F with .50 and above calibres.
I use wet patches for target shooting, mostly WWWF with a tich of Neetsfoot oil (2oz. to a quart). Once on paper at close range, with target at 50, I start load development, going up 5 gr. at a time until the "accuracy" load is found for that rifle and combination. With this process, no wiping is required at any time during load development on any given day. Of course, clean the rifle properly at the end of the shooting session.
Note, do not use a water soluble 'lube' for hunting, where the "load" will remain in the bore for over a few minutes.
I tried to get the pictures of muzzle crowns to re-print, but failed at that. This is the thread I spoke of.
https://americanlongrifles.org/forum/index.php?topic=84556.msg827571#msg827571

Buckstix, for that big bore, .830", a tapered stone with 1/4" or 38" drive stem can be wrapped with emery or wet/dry paper to smooth the sharp edges of the crown.
This will allow easier loading. The ball and patch combo must give some compression in the bottom of the grooves to effectively seal the flame and pressure behind the ball and also reduce the fouling left in the bore.
I feel your choice of a .020" smaller ball is likely a good one as the larger the ball, the more lead must be moved to fit tightly.
Measuring the rate of twist is also a good idea. Many of these older guns have quite fast rates of twist, however some also had very slow twists. The faster the twist, the less powder is used to get good accuracy. The slower the twist, more power is required.
According to W.W.Greener in the 9th Edition of "The Gun", the 8 bore (.835") ctg. load was 10 drams with an 862gr. ball.
This will be an excessive load for a muzzleloading rifle, imho, especially if the rate of twist is much shorter than 100".
Jim's suggestion of 150gr. to start is, I think, a good one and to proceed as you see fit.