Plan A will have two MRBF in series with the right hand battery, one MRBF in series with the left hand battery, this may cause unequal charge/ discharge current.
Check on quality of isolator switch, at high current, voltdrop can be high.
Ensure mechanical fixing of switch and shunt to avoid undue stress on the structures.
Here are the specs and a link to Will Prowse's testing: Discharge information:
Maximum Continuous Discharge Current:200 A
Peak Discharge Current:800 A (7.5 s ±2.5 s)
If it were me I would go with 2/0 cables on plan b and that should work good for you and be a little more flexible unless you're going to be running at max amps all the time