I have Cummins Turbo Diesel in my 1949 One Ton 3800 and I have only one batterey in the original location under the floorbord! Can't figure out why you think you need two batteries?
Chevrolet saw fit to put batteries under there for many a year! The location on the firewall that is stock will hold one good Battery that is all you should need! I have at least 6 of these Cummins powered vehicles and they all have one battery? They start immediately even at zero degrees?
