rb25 and rb26 rods are almost identical, except rb26 rods are beefier so using them with rb25 pistons is fine as long as you're using the rb25 crank. the rods arent the issue.
to use rb26 pistons in an rb25 block, you'll need an rb26 crank. no machining is required to do so, and ive seen some floating around for around the $150-200 mark.
the rb26 has a larger throw, which is the reason you've got the .1L extra in capacity. using the rb26 rods on an rb25 crank will give you really shit compression as the throw is smaller.