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

If the your current heads are in the 70-72cc range, bolting on the Ede's will get you about a 1 point increase in compression.

Even if they flow the same you'll see a benefit there. Do you know what your current heads flow through the range?

Stock J heads flow in the 200 cfm range at .500
Really nice, fully ported J's go in the same range as stock ede's.
Eddies flow in the 250cfm at .500 lift out of the box. Do some very minor clean up and back cut the valve and they will be in the 260 range EASY.

Better flow, more compression... I'd say you'll notice a difference

Here's a good site for some flow data. How accurate???
http://users.erols.com/srweiss/tablehdc.htm#Chrysler