Your round balls will work just fine but don't expect optimal accuracy and you are going completely off the reservation so all bets are off.
At roughly 200gr each, I doubt you'll be able to get more than 2 in there. You'll be looking at an overly long for bore 400gr projectile.
Powder wise, I'm thinking very bottom of the chart for 500gr bullets, with the fastest burning powder on the chart. No matter how hard you get it, your lead balls are going to be softer than a jacketed bullet and have less bearing surface so pressures should drop and velocities will jump enough that it may surprise you. You're going to be in the realm of 45-70 Trapdoor power levels, so this is still not a kiddy/plinking load.
Just my 2 cents,
MLG