So you are going to build a cannon to fit the mould, is that right. Or, are you looking for a mould to fit a cannon?
Maybe I can help you, I have moulds for 6 pdr round ball, beer can round ball, and golf ball round ball. These are steel moulds for casting zinc.
If you cast lead in these diameters you will have to load down to reduce pressure, that may not be all that bad. The other down side to lead round balls in the moulds you are looking at, the balls may not come out round. In fact the hotter you get these mould the more oblong they become.
The steel round ball molds once they are used for zinc can't be used for lead...they just won't come out right.
If you shoot right you can go down range and find your round balls and recover them. Most of the time you won't have to recast the zinc balls, you can resuse them. The lead balls will have to be recast everytime.
If you shoot heavy loads, loads that go boom, make a big cloud of smoke and the balls just disappear, well that dangerous; it stresses the gun and you loose the balls as they land, you know not where. Remember always know your backstop. If you don't know exactly where you ball is landing that is dangerous.