I would stick w/the 2 1/4 pipe on the 318,that motor needs a bit more back pressure.On the other hand if you,re gonna upgrade to a 340 or 360 in the future,go with the 2 1/2 now.Also, when you install you,re alum. man,i went with a wider stainless washer under the bolts to distribute the load more and the stainless won't rust.You may have to check you're torque a few times after the engine is cycled a few times from cold to hot,etc. The bolts tend to back down a bit after a few weeks.Glen