Going Carb does not mean you have to change your ignition. There 2 seperate things. If you buy a older motor that already has a carb, than more than likely you just need to run the fuel lines over to the fuel pump on the passengers side. To convert your 305 to carb.
Change the manifold (You can use the new manifold on a 350 when you swap)
Get a Fuel Pressure Regulator that allows for fuel return (If your not going Mechanical)
Get the Carb
Get a inline Fuel Filter