My throttle pedal on my 340 seems awfully hard to push giving the car a heavy or slow feel. I have the factory return springs attached at all locations. I recall another post with the same symptoms but can't recall any fixes?

 

The accelerator pedal has a hinge at the base which can become stiff and even rusty. Have you tried giving this hinge a good does of oil and a working with your hand to free it up. This if not treated could result in a stuck open throttle if the pedal does not return.
There are also a couple of rubber bungs on the throttle linkage where it goes through the bulkhead. These can become worn with age and usage. The rubber can become stiff with age and they might need replacing.
Lastly if your car is an Auto when you depress the accelerator you lift/pull the kickdown cable up. If the Bowden cable is stiff this will effect the feel of the pedal. Oil down the cable to loosen it off might help. You can check this one by disconnecting the cable from the throttle linkage at the top and see if the accelerator pedal is more free.