SD66 is iirc a 66 mm inducer which = a 2.6. 2.5 would have to be somewhere in the 63mm range. If your tech isn't real strict I would think you could get away with the SD66 until you start beating everybody. LOL
What are the rules in your 2.5 class. Because every 2.5 class I have seen is just that. Doesn't have to be a stock appearing. Just a 2.5 inducer charger. An s464 would pass tech.