i just got a 3.3 for mine also ( i had the 2.5 so its a direct drop in)
but this is what you need

[link=http://www.amainhobbies.com/product_info.php/cPath/1_25_1466_660/products_id/17771/n/Traxxas-Nitro-Rustler-Pro15-to-TRX-25-Upgrade-Kit?utm_source=Google-Base&utm_medium=cpc&utm_campaign=Product-Feeds&source=google_ext]Click Here[/link]

Details:
part #TRA4400
Traxxas Nitro Rustler Pro.15 to TRX 2.5 Upgrade Kit
PRICE: $22.49


All the necessary hardware to bring older Traxxas models up to full TRX 2.5 spec is included such as; header, pipe, pipe hanger, coupler, linkage, and additional components specific to each model. Major components come pre-assembled and pre-adjusted from the factory. Detailed instructions are included for each. TRX 2.5 Racing Engine™ (model TRA5207) sold separately.




get the upgrade kit to add a 2.5 as posted above. because the 3.3 takes the same linkage and mount.