TBI systems can provide a benefit. However if your altering the base engine with larger valves and such, a carb is so dynamic and freindly. I would chose a edelbrock AVS or afb around 600-650 cfm. The AFB/AVS is easy to tune, rebuild, and very staight forward. The intake for pulling would stick with the performer 318 to keep continuity of the small head runners. It will stimulate the low end for pulling and towing.
If you want more on the Magnum swap look into Moparts.com, and make this post. You can get a pair of magnums around $179.