Enlarging the flash hole is no longer recommended but if it works for you just keep those cases seperate from the ones you use for smokeless.
Spence Wolf wrote the book on replicating the Springfield load and much of what he wrote is adaptable to all black powder cartridges.
http://www.the45-70book.com/pat
If you decide to go with black powder, it would probably be a good idea to forget much of what you know about smokeless reloading. BPCR is a fun time but it is also a learning process. I have simplified things tremendously since going to black powder. I am no longer a competitior so I do not try to wring out the last .000001" from my rifles. It can be done but not by me anymore.
This is an oversimplification of how I determine the first load in a new cartridge or rifle. Lay the bullet of your choice along side the cartridge and note where the base of the bullet would be in the case when seated properly. This is where you want your powder column to be. I fill the case with powder to that point. Weigh that amount of powder and dump that charge into a small vial. Slowly pour the powder back into the case though a drop tube. This settles the powder in the case and allows you to insert a wad or two to fill the void and protect the base of the bullet. If all goes well, weigh out another charge and proceed. I usually load five or maybe ten cartridges this way and test the load. If this load works for me, I make a measure to repeat the amount of powder. Some of my measures fit the powder can, others a flask and some I use to dip powder from a box. Regardless, I always drop tube the powder into the case to settle it.
Once you get into it, bullet alloy, bullet lube, wad material, lube wads, seating depth, paper patching, patching material aned etc all make interesting areas to explore. Reams of paper and gigabytes of information have been used in the detailing of experiments. As I get older, I try to keep things simple but it was fun to experiment.
Start simple, use a good black powder lube and have fun.