The current tank that is being shipped with the kits have been upgraded and are larger than the originals. I contacted Tony Z at one point and he sent me a new one. I mounted mine on the firewall similar to yours. The setup in this kit is not a closed pressure system due to the radiator, therefore you need an overflow, I ran my overflow line from the radiator to the overflow tank along the frame but hidden behind the body sides. Once you have the system full only put no more than 1/4-1/3 full additional coolant in the tank or it will overflow until it stabilizes and you've run it hot a few times.
20180331_144720.jpg 20180410_191053.jpg This pic shows the overflow line going from the rad to the reservoir.