Hi, Xklaba -
I used to have examples of the stock RB25 and RB26 covers in my shop, and they are different, sadly. The profile of the 26 is curved, where the profile of the 25/26 is more like "faceted". I'm not sure the bolt pattern is different. If I were you, I would find a shop that has the 25 and the 26 covers, stock, and compare them for similarity of bolt pattern before I would try to bolt a 25 cover on the front of a 26 engine.
Actually, if you could find the stamped sheet metal piece that bolts to the front of the RB26 head, and a stock RB25/20 front cover, you could verify the bolt pattern that way. Great question!
- Rob@Zaklee