25's are double synchro just like the 26, both boxes use the same synchro's, bearings, rings, etc
internally the only visual difference i saw was the input/main shaft looks a little different, 26 is thicker in some places, i've been told that the gears are the same as well...
there may be other little differences but basically the same,
25 boxes are very rare these days, drifter love em, so you could probably buy a 26 box and change to internals over to the busted 25 box, was considering this recently but decided against it, by part numbers most parts are the same but making parts fit is a different story...
has any1 tried this? ive heard a few but no one willing to share secrets