most 231s are "even fire". i believe it was only the first year and a half or so of production that used the 225 "odd fire" pattern and crank.
given that you want factory fuel injection, that's going to mean looking for a 3.8L designation as Buick did away with the 231ci nomenclature in the Seventies and FI wasn't implemented until the mid 80s.
in fact, don't think GM offered FI on the "3.8L" displacement
unless you're going to with a Turbo v6 from a mid-80s Regal!
otherwise, if you're REALLY fixated on this FI idea, you're actually going to be looking at a newer engine in the 3800 series.
there is also a 4.1L displacement from the early 80s, but those are all carb engines. it's the LC4 on wikipedia.