The RB25 and RB26 blocks have the same sump bolt patterns. You can bolt either sump (RWD-4WD) to either block. The 4WD sump needs a different crank girdle, there is a relief in the casting to accept the front drive shaft. You could mod a RWD girdle to suit.
You'd think that but the 4wd blocks are totally different and they get a huge amount of extra machining.
Trust me, the 2wd blocks do not have the extra casted area along the sump rails or the mounting holes to mount a 4wd sump, hence the need for an adapter plate when wanting to run a 4wd sump on a 2wd block.
You can put a 2wd sump on a 4wd block though as they do drill and tap the 2wd pattern on the 4wd blocks.