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I had a similar problem with mine. We had to change the springs in my distributor to re-curve it and it fixed the problem. It might work for you.
It's the power valve. The carb is your whole problem. Run 71/81 jets with the rear power valve blocked off. timing at 38 total.......unhook your vacuum advance. If you have to turn the idle screws out more then a turn and a half then the idle circuit is to lean------a common problem with a cheap carb. If i put my 750 holley hp on that 340, your plugs would look like they where run in a fuel in injection engine they burn so clean. That cam is a great cam by the way. There is a trend --- as of the last 10 years or so --of over estimating a cams ability to make more power then the next grind. When your in the right ball park as far as your cam choice -- not to big or small moving to a bigger or smaller cam isn't really the problem solver you would think it is. --the rest of the car is what makes your car faster then the next guy.
I ran the 484 cam in my 70 340 4sd. Duster. Installed it at 104 ran a Mallory unilite that was 20 degrees in at 2000 rpm and initial @ 18 . LD-340 intake 700 DP 3.91 gears. Performed very well and I never had to retune. Just had to keep replacing the rear tires!
I have that cam in my low comp 360 in my Dart Sport which is stock other than the cam, weiand dual plane intake, and a 600 edelbrock avs carb. Everything else is stock including exhaust and it runs well and is very driveable with 3:55 gears and a 4 speed. The initial timing is set as high as the starter will allow, and it is a good driver that sounds good and is very smooth. It runs at 180 degrees all day in the heat. It's not all that powerful, but not bad for a low comp 360. Advancing the timing made a huge difference on how it performs. After you get your initial carb settings done, timing is the first place i would start.
Might check the fuel pressure at idle and higher rpm, haven't seen that mentioned.