The best way forward is to remove the kidney box, BUT this should only be done if you can feed the oil return to the sump.

If you return the oil to the sump, it will then free up the oil return on the block to act as an additional breather to your tank. Dont bother to breath from the rocker cover.

The problem with returning the oil to the block is that there is pressure in the crank case whilst the engine is running, so it can only drain when the engine is stopped. There is no pressure in the sump, so it will be draining whilst running, meaning the after market tank will not fill up causing excessive oil blow from atmosphere pipe on the set-up.

Hope that helps.

PS - You can get a connection to replace the sump plug with an oil return plug.