If you're looking to use one of those aluminum type tanks they come in 13", 15" and 17" heights. The 15" fits nicely along side your existing radiator with the mounting tabs lining up with the radiator mounts. There are a couple of hot rod web sites that offer them at around $35 delivered and they appear to be the same ones that can be found at twice that price at some of the Mustang vendors. Note that they are a bit small in capacity so on really hot days you will likely spill some coolant from the tank overflow.
Good question. It's HOT hear in Oklahoma. I am tired of stalling out because the soda pop bottle overflowed and sprayed down the engine with coollant. Thinking about visiting the local junk yard for a good one.
Try the 15" overflow tank from RodWorx. It's cheap and effective, and sucks the coolant back into the radiator when it cools off, thus making the capacity question somewhat moot. I've yet to lose any coolant out of this overflow tank, and I've had a long history of cooling problems. I don't think it'll be a problem.