The Carter or Holley 2bbl. carbs might not bolt down to the Stromberg mounting points. This I am not sure of.
The OE intake in a 4bbl. style is a little hard to find, but, like said above, keep an eye on E-bay, they will pop up. Due note that only a carb with dual mounting points (Current Edelbrock carbs) or OE Carter 4bbl.'s will fit since there is only one set of holes to mount the carb. Holley carbs will not bolt on the intake.
IMO, the 500 is a good choice, a 600 will work just fine as well as it is my personal choice for use on a stock 318. (Year unimportant.)
The OE style Orange box is fine. IMO, a MSD will gather more mileage and fire off more of the air fuel mix for full power potential. (I have both operational in my car(s) in case of MSD failure.)
Make use of a modern electronic distributor with a vacuum advance unit. A newer 318 distributor will bolt right in without issue. I have no exp. with the Petronix electronic ignition.
After your done with this, the next point would be dual exhaust off the manifolds (Or headers if you run into some decent coin for tti headers) @ 2-1/4 inch at a min, (2-1/2 max with added performance parts, if you go that route), use with a cross over pipe or X pipe.