The B.O.P th400 will bolt up to the 231 no problem there.
But the transmission mount has to be made to fit or buy one from a vendor who sells them that will allow it to bolt up to the crossmember.
Another couple of things that needs to be addressed is the driveshaft length,some have said they did it without cutting it but i didn't want any problems so i had DENNY's driveshaft make me one the correct length and front yoke installed, if you do use your old driveshaft the front yoke has to be changed for one to fit the th400 also the vacuum modulator needs to be either modified or replaced with a mechanical modulator (that's what i did) or use a manual valve body in the transmission.
The reason the bell housing looks different, is that the BRF code 2004R is a dual bolt pattern transmission. (and the TH400 is only a BOP bolt pattern) In that it will bolt up to a Chevrolet AND a BOP. (Not all 2004R's are dual bolt pattern, though.)