You have a good intake for that engine. Definitely keep it.
You cannot put larger valves in the 273 without having to notch the bores.
However, you can improve the flow with those heads with some good porting in the bowls and blending it into the gasket matched ports.
Then if desired, gasket match port the intake manifold and blend it into the intake ports.
Also, it is not good to mill a closed chamber head. With the smaller combustion chambers you will not gain as much benefit as milling an open chamber head. The closed chamber head also does not have that much volume that it can afford to loose before "washing out" the chambers. They have issues with "wash out" on open chamber heads where the combustion chamber was milled to far down and it looked like a closed chamber. Those were not allowed to be used. There's much less risk in milling an open chamber head than a closed chamber.
