Will,
I haven't had a transfer tank, but it sounds like Bulldog gave you good advice.
As for tool boxes; I (or my friends) have had several different ones. The old style that hinge in the middle are a pain to get anything long into. Plastic ones usually don't last long, and will sag with weight in them. This shouldn't matter, since you will have a fuel tank under it. Steel or aluminum will work, I would buy one that matches the tank you get.
The design I like best is the sinlge lid that has the hinge towards the cab. Make sure that the fittings are well made, My friend has one that the catches gave out on. I have a steel Delta that still works good after about twelve years, except the piano hinge is leaking rust.
The best tool box that I have ever had came on a used pickup, it is a steel Weatherguard, made by Knaack. They are expensive, but well worth the money. The push button locks actually work, and really lock. No rust, and the contents stay dry. I would not hesitate to buy another one.
Hope this helps,
Robert