The single Holley Weber 2bbl is relatively inexpensive and easy, a few guys I know who did it, including Tom Langdon, reported increased full throttle power and improved fuel mileage, but a reduction in part throttle power due to the carb's small primary throttle barrel.This may not be true in every situation, and some may not mind the trade off.
Carbs on adapters are a crompromise at best.I used an adapter to install a Rochester 2GC V-8 carb on a 261 Chevy 6.The engine was stronger at part throttle, but not full throttle,re jetting made no difference.I suspect the adapter necking down to the one barrel carb opening was messing up the air flow.
So I took a spare stock intake manifold,cut out the area around the single carb opening, made a mounting plate to accept a 2bbl carb.I used a Holley 2300 of around 220 cfm cause their readily available and easy to tune.Now this worked real nice, improved part throttle response,better top end power and better fuel mileage.I'm over simplifing the modification,but it was done with common hand tools , not all that difficult, and others have done it using all sorts of carbs.
Holley 2300 on stock 261 intake Years ago, racers used to flip the stock intake upside down,cut out the bottom,braze or weld on a plate to accept a four barrel carb.The carb was tilted at a little funy, but it was ok for circle racing.You can also block off the stock carb opening, and weld on two mounting plates , turn the stock mani into a 2 carb set up for cheap money.
This is the fun of these old inline engines, making your own speed equipment like hot rodders did 50 years ago.
Bob