You need two "must have" books to study: "Building Double Rifles on Shotgun Actions" by W. Ellis Brown, and "Shooting the British Double Rifle" (2nd Ed.) by Graeme Wright. The former gives you fantastic step by step building instructions with profuse photos, plus regulating and gun-choice info. The latter is the Bible I believe, as far as BP double rifles and nitro-for-black loading. Wright also gives a lot of pressure data.
I, too, have studied the BPE British doubles considerably, and regulated 5 or 6 so far. My opinion is that for all around hunting not to include rhino, hippo or elephant, create or buy a .500 X 3" double. For approximately 25" barrels, convergence of the axis of the two bores from breech to muzzle will be about 0.2", to best of my recall. If you place the barrels on a surface and sight thru them at a spot 100 yds away, the two bores will be "cross-eyed" by about a foot to best of my recall. In other words, the left barrel bore axis will be pointing to a spot about 12 inches to the RIGHT of the right barrel.
I very recently chronographed a .500 X 3" BPE double made in 1876. 138 gr. compressed 2F KIK BP and 364 gr. bullet gave avg. of 1725 fps for two shots. Recoil is somewhat above modern .375 H&H rifle, but not that of the .458 Win. Mag. In short, tolerable, suitably accurate, and straight case - deemed better by both Sharps and the British makers in comparison to necked BP cartridges.
.450 X 3 1/4" is a close second but not quite the oomph. Easier to find an old one than the .500 X 3".
Aloha, Ka'imiloa