The side of the anode pointing toward the cathode counts. The back side not nearly as much.
I can't say I've ever seen anyone running more than about 20ma with a 1ozt silver bullion bar though But then that doesn't have 6 sq-in surface area on a side so you have to be talking about a larger bullion bar, right?
Even if you can, at high currents like that you may well need a stirrer and for sure would need to process at a higher temp (I assume you're talking about higher than 20PPM - I can't say if you could do 20PPM cold at that high a current).