I think I'd ask Billy Marr about the bullets, he can probably fix you up. As for throat dia., just take a fired case and fill it with lead. Chamber it, drop a pure lead slug in the muzzle that will fit down the bore (a smaller caliber bullet, or fishing sinker works well) and upset it into the throat using a close to bore diameter mild steel rod. You don't want it to be too small, or it'll bounce around that scratch the barrel. Deburr the ends of course. Hit it with a heavy hammer until it bounces and you can feel the slug is fully formed to the throat. Open the action, extract the case, and carefully push the slug out. You now have an impression of the throat, so you can check it for diameter and length.
As for 7383 powder, it's used in the 50 spotter round. Not commonly available, but you can get it from Pat's Reloading. Don't have the url but a google search should turn them up. It's dirt cheap (like $2/pound) and works well for many medium bore cast bullet loads. Burns about like 4350, but is way bulkier.
Keep experimenting, and I'm sure you'll find 8MM happiness once you get the combination right!
