I have a narrow grooved .32 in which I use a ball that is .009" smaller than the bore and a .0215" ticking or .0225" denim patch. Those measurements are with a mic. My calipers makes the denim out at .025" thick. This seals well, but then the grooves are only .008" deep.
I would find some 12 ounce Denim and try that. It mics .025" and .030" on calipers. I'd couple that with a .451" ball for the .460" bore. Lucky, ther are a number of factory moulds, Lee and Lyman right around there - 451", .454" and .457".
Good luck, those deep grooves could be difficult, but a .460" bore + .-016 + .016" = .492"- that's groove to groove.
.451" ball + .025" + .025" = .501". Theoretically speaking, that should give .0045" compression in the grooves each side and shoot well. It's where I'd start.
I use that same patch with a ball that's only .006" smaller than the bore, with .012"rifling. Yes - it takes a good single smack to get it level with the bore and another on the starter to get it down a bit so I can use the gun's ramrod, but I find it not bad loading at all - for the entire trail walk. That combination should work fine in your barrel, Pt. Lon.