I have a silly question, why can't you download the 4.3.1 firmware and hold shift to select the firmware from iTunes.
Unless you had saved your SHSH blobs for 4.3.1 you wouldn't be able to do that. Apple is only signing the newest firmware release 4.3.2 which prevents you from installing any previous releases (unless you have the SHSH).
My golden rule is save those SHSH right away. It's the very first thing I do with any device and any new firmware build.