So i completely forgot about the coolant expansion tank untill i brought my radiator into work today to have them modify my outlet. One of the guys here used to own his own open road racer, and mentioned how he thought it was way too small. The opening on a 33 ford isn't very big so i didn't have much of a choice. But then he said if i didn't have space put my overflow against a frame rail, and i responded with an "oh ****

i forgot about one of those"
So how big of a tank should i use. I figured a smaller radiator means i should have a bigger tank, but im crammed for space as it is so its going to be tricky.
Also can it be mounted below the radiator? Its pressurized correct? so gravity shouldn't matter??? Any info could be great.