Im not sure about the kits, but is your truck using an actual diesel rated battery? It seems that many parts stores are selling a non-diesel battery in place of the OEM type replacement.

Not sure why, but as an example O'Reilly's doesn't even list a battery for the 93 CTD trucks. So they often just sell the group 27 battery to customers.

I don't know if there would be room on the passenger side near the air box or not for a second. My truck had a second battery on the frame rail under the box, a fabbed battery box was attached to the frame, and lead wires ran along the inner frame rail, up to the main battery.