73 Duster Twister Guy

Will be saving and preserving the 318/3spd. Grew up on 8.75 rear ends & not familiar with the 8.25 for strength or durability.

Welcome to FABO, Ted!
The 8.25 was fine behind a stock 318, but if you're planning a 'healthy' 408 I believe you're going to need to replace it with a tried-and-true 8.75 for longevity.