Just to clarify-- you want to join the batteries together with a bus bar, right? Anyway, it seems a 3/0 cable is rated to 200 amps at around 160 degrees F. Hence, replacing it with a 1000 amp bus bar should be fine. Since you say the bank capacity is only 400 amps, you could even go with something smaller.
Find an electrical distributor Ilsco Products .They make one that is insulated and I'll promis you that it will be a lot less than 60 dollars .I believe the cat number would be a PDB3/0-4 .