More than likely it was used on a small displacement, high RPM, extremely efficient engine. 360 cubic inches or less. Bigger is not always better... Not to bash the OP, but I would take the top port over the bottom one any day of the week, chances are it is a very highly efficient, high velocity port with every aspect of it set up for a max effort combination. The short side radius is a lot bigger than the bottom port for sure. The bore size wouldn't be as critical with that port it as long as it was 4" or more.