Carb selection?

Too big a difference in cubes and requirements to expect one carb to do a good job for both engines. The 408 will want something in the 850-1000cfm range - just way to big for a 318.
The Holley 650hp would work good on the 318 and give you some experience with tuning a holley for when you get to the stroker - and a 650 can always be sold for a good buck as they are a size that is always in demand.