I have a 1969 Barracuda and need to replace the carb. Besides changing the intake manifold is there anything else I should do to replace a 2 barrell carb for a four barrell? What would be the best way to do that type of switch?  

The best intake for this swap is the Edelbrock Performer intake. I would recommend a 600 Holley. You will need to make a fuel line from the pump to the carb. Don't forget to reinstall the throttle kick down for the trans. which hooks to the carb linkage.
Another option is to source an intake & carb from the wrecking yard for a 318.