Which carb should I get?

I would go with the 600 whether it is the Edelbrock or a Holley. The Avenger comes with fittings and rubber hose for the dual feed so that shouldn't be an issue with clearance to the intake. Also, there are so many dual feed line kits there shouldn't be an issue to find one that works.

I have a 670 Street Avenger on my 360 which is approximately 370 HP and it works great on that application. The 570 Street Avenger would be a great choice for your 318 too.