I tried that very adapter and a Rochester 2bbl once,it really didn't improve the performance and the adapter caused mixture issues at wide open throttle.Of course your results may differ.
What did work well for me,I cut open the original carb opening on the intake,reprofiled it to match the two barrel I was using.My grinding got into the double wall area of the heat riser section,some work with a Mig welder was necessary.I then made an adapter plate from one inch thick phenolic material,bolted it to the modified intake after drilling and tapping some holes.Made up a linkage cross over to utilize the stock gas pedal set up. Wasn't all that difficult to do really.
This set up works very well on a 261 engine,gave more power,better throttle response and actually a few MPG too.I didn't use a Rochester carb,but any two barrel should be ok.
