If you are going from 2 bbl to 4 bbl, you should definitely get a 4 bbl intake and NOT use an 2 bbl-to-4 bbl adapter.
A first option to consider, just get your existing 2 bbl rebuilt. A good shop might do a great job on it and that may be the cheapest way to go.
I have a Factory 2 bbl Holley on my '74 Dodge Pickup. It was rebuilt just before I bought the truck last November, and though I will be swapping up to a 4 bbl intake and carb fairly soon, I would have to say, I was very much surprised at how well that Holley 2 bbl performs, drivability, relative fuel economy, etc, (I just want more Power, hence the future swap).
Another option might be a New higher performance aftermarket 2 bbl carburetor, like a Holley 2300 series, (500 CFM, $313 at Summit Racing), then add the Electric Choke Conversion for a bit more. Then you could just keep your existing factory 2 bbl intake.
http://www.summitracing.com/parts/mll-bpc4931ct/overview/make/dodgeFinally, you could buy an aluminum 4 bbl performance intake manifold and 4 bbl carb. If it were mine, I would go with an Edelbrock Performer RPM Air Gap (if it would fit under the hood) and a Holley 600 or 650 Cfm, but there are many other good carburetor brands out there from which to consider.
Holley 2300 pictured below:
