Odds are very good that the 231 you installed was configured to bolt to a 5-speed transmission which means its input shaft is too short to seal where it enters the auto tranny's output shaft seal. The easy fix is to install an extended output shaft seal into the auto tranny which is sold by a number of sources like either Advance Adapters or JB Conversions (one or the other I can't remember) and Tera. This moves the seal itself further out a tad where it can properly seal against the 231's shorter input shaft.
Here's the one at Tera where I got mine since mine did the same thing after converting my 5-speed tranny to the same tranny you have.
Automatic Seal Extension | TeraFlex Suspensions
And a big welcome to JF!