Besides the mould you will need another 80-100 bucks for platinum lined nipples.
If you use enough powder to make any velocity you will surely exceed proof load pressures in this gun with a conical.
As Daryl stated the gun is designed for the RB and will perform better on game with that projectile. If you intend to shoot African Elephant its marginal with any projectile.
If for some reason you are worried about penetration it will probably shoot WW balls ok and this will greatly increase penetration if its needed, though outside Africa I can't see penetration being an issue with a 12 bore.
The current ML "conical craze" was started and fueled by companys making and selling various conical bullets. Since they made/sold conicals the RB *must* be deficient or there is no reason to buy their product. So the round ball is bad, conical good. Facts and historical data is irrelevant.
If you give this rifle an honest trial with a RB you might be more impressed than you think. Its real killer and capable of taking any NA game with a patched RB though a W-W ball might be the best choice for large bears.
With a reasonably tight cloth patched round ball the projectile in the unfired barrel will likely not move under recoil. With a "naked" conical you will need to reseat or at least check the bullets position every time you fire the gun to assure you don't have a spaced bullet in the unfired barrel.
As Daryl pointed out this is NOT the gun that the old English 12 bore rifles were. It is not breeched the same and lacks protection these guns offered the shooter. The location of the nipples, the hammer shape and the fences around them were all carefully engineered in the good grade vintage guns so as to protect the shooter.
As Daryl said it is certainly your rifle and you may use it as you wish but you should do some research and some shooting with the PRB before trying conicals in this rifle.