390 vs 408
I find that the small bore of the 318 might have an advantage over a 408. This smaller area is capable of holding fuel and air molecules together, tighter, therefore increasing flame propagation and flame speed, increasing in cylinder temperatures, all this helps make more power and economy as more fuel is burned during the process. A head with a good combustion chamber and engine tune would be necessary to take advantage of this scenario. This can also help decrease the chance of detonation.