Stock 340 intakes

That LD340 is a kick *** intake and will stand toe to toe with a RPM up to a point. Your in that zone where it will perform right along side the others.

The 340 cast iron intake is not a bad intake. It is limited in RPM and max power output by compare to the other aftermarket intakes. But not all of them.

Some things to help mileage;

K&N or equal filter, an extreme top is a nice add on.
Take the time to really tune the 600.
4 hole spacer
multi spark ignition
(I've added Rhodes lifters to an engine once and found more torque and a slight increase of mileage. But thats rather extreme to do, it was I think $100 for the lifters and then gaskets and time, though everything here is only a little bit to each mod. When it adds up is the goal to reach for. The MSD is the best mileage mod I ever did, minus the Jacobs which was better.
The intake change could very well add alittle mileage.)