"Is there any dis-advantage to using the 42mm carb with out a flow job..."

Yes, it costs more. The 38TMX is more than enough for your application. You'd be wasting your money on a 42mm.

 

A400l is right.

Unless increased airflow is called for by the pipe (cam) and ports, a larger carburetor is just a waste of money. Your fuel mileage will go down, venturi velocity will go down and so will your performance.

Ask anybody who ever wasted their money on a 750 CFM double pumper Holley on an engine that couldn't handle the increase in CFM.