starters are that the head bolts are different sizes ...
id be interested to see how you would block the vct oil feed in the block and the rb26 doesn't use it..
other than that, without knowing what car its going into its hard to say how much trouble you might have .. but the rb25 ecu should eb able to run the rb26 head, just without the intake temp sensor working.
im sure you could do it, but for the cost of an rb30 bottom end and the PROVEN power from that, all i can suggest is to go that route..
nothing is usualy impossible, just impractical.