For starters you need to know what rear end ratio you have now and whether you can live with that. The 7 1/4 might be just fine behind a 318, very much depending on what you are planning to do. A 8 1/4 will be stronger... but again we need to know your plans. Mild 318 or more performance oriented? When you tell us your plans people can give recommendations on the rear end, but then I would not start this build with changing the rear end. :)
Also have a look in the "How to"-section or use the search function to look for /6 to 318 conversions...
