Eddy heads - will I feel a difference on my 340 ?

I think it boils down to your wallet, and your butt. If you drive the car hard enough to notice small improvements (like after you change plugs, or clean the air filter) then I'm sure you will notice a decent improvement. However, if your heads are done really well, and the Edelbrocks show typical production line issues, the gain you feel may be just the weight loss. In any case, I am sure you'll feel something. Only you can judge whether it's "good enough" to justify the cost. Real good iron heads cost a bit to get. Same with aluminum, but edelbrocks are starting from a much better point thatn the iron.