You may want to do a search on this. BP is way diffrent than smokeless, for one thing you can't just dump 70 grains of black in the case, It won't fit, You have to use what's called a drop tube to get it in there. It helps the powder to compress and pack into the case. 405gr bullets are the basic round used in the .45 GOV but you can go up or down in bullet wieght, are you gonna use cast or jackets? Makes a diffrence in loads also, if you are gonna use cast you need to use bullet lube for BP not smokeless.
It really isn't hard to do but I would recommend you read alot and find out what you need before trying, Its fun to do I like to shoot it in mine sometimes but remember this BP blows up if handled incorrectly, Smokeless burns really fast it dosn't really blow up, That fact makes BP more dangerous but only if you don't mind your P&Qs