you want the bigger classic that is for the special hks kit which is 2446cc or something like that with a custom crank the diagram is an example of what you need to do there are so many variables out there of the build and Ihave heard many comment about most .
but for the rb25 I think the rb 26 crank and rods are a good idea.
for an rb 20 you are just better to put 82 mm pistons is and make it a 2.2 as it revs much better
cheers
meggala