twin carb setup would be better, the 350 holley's really aren't suited for red 6's without pouring tons of money and petrol into them, more faults than they are worth!
as far as which one is suited size wise would depend on what works done to it but i reckon for set and forget the holley is the way to go ... my experience with multiple carbys is they are a pain to keep in tune
A 350 jetted correctly will give you good performance and economy. In saying that, as long you don't have a wild cam fitted in your block. Thats where the tuning will start to give some headaches.
