There is no need for an engineers plate(afaik) because r32 did come out with an rb25 as well.
but, that being said, i would recommend it anyway, make it look more legit and less likely to be picked on for mods if its been done right, and approved.
as for the rb26conversion.....that is a great engine to throw into a gts-t....untill you break it, then you'll wish you got a 25....they are soooo much cheaper to fix/replace.
6k would get you a weapon 25, or a stock 26...
also with the 25 you can use most if not all the boltons you already have...with the 26 you are starting from scratch.