Spread Bore Pro's:
-Smaller primary bores offer better throttle response and better fuel mileage on a well-tuned carb.
-Large secondaries allow the primaries to be tuned a bit more on the lean side for fuel mileage and the secondaries can be on the rich side so at full throttle you get a good mixture and at part throttle you get good mileage. Secondaries provide the bulk of the flow at full throttle so you can get away with doing that.
Spreadbore cons:
-lack of choice of aftermarket intake manifolds that support this pattern. It is much easier to build an engine around a square bore than a spread bore just in terms of intake selection. And square bore -> spread bore adapter plates are said to hurt power output.
-square bore carbs in theory provide superior air/fuel distribution. One carb in the center of the engine with all intake runners leading to it, it only makes sense that a symetrical design would be best for full throttle performance. This requires a little more careful tuning because at full throttle you want all 4 barrels to flow at the same air/fuel ratio but at part throttle want the primaries lean enough for decent fuel mileage. This often just doesn't happen and is why holley double pumpers have such a bad reputation for fuel mileage.
-lack of aftermarket support for spread bore carbs. Square bore holley and edelbrock carbs are very mainstream where the spread bore parts are not as easily available. There are guys getting good performance out of thermobogs and quadrajunks but again, just not as popular/mainstream as the conventional square bores. Holley did make some spread bore double pumpers and vacuum secondary carbs over years but they don't have much popularity these days.
I have come to prefer the spread bore, my thermoquad provides me with good power and good fuel mileage. I can actually do a lot of putting around town on 2-3 gallons. But the important thing to remember is any carb can be a fuel sucking, performance robbing turd if not tuned well.
