Pete1980,
I thing that your build looks very good so far!!!
I like very much the traditional look 2 toolboxes - 2 fueltanks and now you have 2 chainracks. I believe that you don't need anything more than these. And as it mentioned above by Tim, an SBFA with Big sleeper should be a cool truck. A heavy long hauler!!!!
I don't thing that your build needs a third fuel tank. Save this tank for a future project. Just thing about a nice headache rack, it should look realy nice with the chain racks.
About your questions, a working truck can be equiped with all these goodys that the operator needs to do the job. If there is not every day necessary to use chains one chain rack is ok, but it is more practical to have one for each side of the chassis. Additional tanks should have their own dimentions in diameter or in leght. Its all up to the needs of the operator or the needs of the job that the truck is used for.
Hope this helps...
Greeitngs,
Kostas.