Rebuilt as follows: had to bore 0.030 over, had to replace factory crank due to hairline fracture, replaced with rebuilt crank casting #2658278 from Standard Crankshaft *** x-heads, Cam MP4452761, original damper, rebuilt trans with new torque converter. Have replace converter which helped a little but not much. Did not have machine shop balance as they were not set up to do so but pistons were supposed to be a balanced set, etc. We are working on weigts on the flywheel and if that works will weld them on converter but this is hit or miss process. Any ideas out there??? Thanks!  

Your replacement crank is a forged crank, your original unit was probably a cast crank.
You will need to replace your harmonic balancer with one for a forged crank and there should be absolutley no weights on the torque converter.  

Good thought but one we have already investigated to the max. The original crank was supposed to be forged as well from everything I have read as they did not start putting cast cranks in the 340's until late 72. It had a neutral converter as well as a symmetrical damper with no weights or "weight removal" on it (the one that is on there is the one it came with, not a replacement damper), meaning it is supposed to be internally balanced (forged crank). I agree it was supposed to be a neutral converter but a 20 gram weight helped to some extent. I am wondering if the crank was not balanced right or maybe the pistons were not a matched set???  

Pistons, highly unlikely, the problem will certainly be in the crank, I gurantee you more than 50% of all 72 340 had cast cranks, plus if you haven't owned the car since new who knows what was in there.  
Any chance you still have the old crank, it might be worth your while to have it machined and re-installed.
Balancing the crank you have is likely the only other alternative.
Why would it need to be balanced, who knows but I can assure you they don't always get the care they deserve.
Someone could have left it laying around on the floor for a number of yrs. which is a no no, they need to stand up straight otherwise they can destort.  
Someone could have actually knocked it over, dropped it, there are a number of ways to damage a crank and not even know it. Not all factory cranks got the same TLC either, some were only balanced marginal at best.
good luck  

Thanks and agreed on the pistons. Unfortunately, dont have the crank that was taken out. I did see the hairline fracture in front of one of the journals however when it was still at machine shop, that is why I replaced it. I would have thought if it came with a cast crank, it would have had a cast style damper (externally balanced). Everything that was on the motor points to it is supposed to have a forged crank *** It is amazing how the documentation from that era only confuses one more. You read one thing on how they were supposedly built, then read something else a day later! I am an old MOPAR nut and love them but have never had this problem and have rebuilt several of them. Thanks again!  

Forget about the harmonic balancer, does the torque converter have a weight welded on it, if so then your old crank was cast.  

The one that came off the car did not have a weight. It was neutrally balanced.  

Then the original crank was also forged as you thought.
I would be contacting the place were I bought my crankshaft and asking for another one. It is not unheard of to get a bad crank from a rebuilder. Like I said it could of sat for yrs. on its side in someones garage, you just never know. This stuff is 35 yrs. old and anything could have happened to it before you saw it.
If the rebuilder won't replace it, then you should source out another, or have it rebalanced.

good luck  

Doc, I believe I have to agree. The best shot right now is to get it the best I can (and it is not horrible anywy) and pull the motor back out this winter and do what you said. Thanks for your input!  

The 278 part # is for a 273 steel crank which is heavier than the lightened 340 steel crank #357 [off the top of my head].
340 cranks have lightened rod journals/throughs [ to rev & for the heavier rods & pistons,] while 273's do not.
TIME TO BALANCE THE ROTATING ASSEMBLY END OF STORY.  

crank might be bent, 340 crank is only slightly lighter than 273/318 truck crank as only 1,2 and 7,8 throws are lightened. did U upgrade to a heavy duty aftermarket flexplate such as B+M , some have a weight on them (for a cast crank and a convertor without a weight). drop the oil pan : forged steel crank = use no external counterweighting, cast iron crank needs non-neutral balancer and a weighted flexplate or convertor.  

More reason to get the proper crank from the rebuilder.  

I checked:my 1970 340 FORGED STEEL crank has forging 2532457 on it, probably the same as 1971 and maybe 1972. it is very simple-you say the damper is not counterweighted=forged steel crank,internal balance,nice(a 4 speed most likely? some say early 72 forged, late 72 cast, I say mopar history shows 4 speed gearbox=forged crank,automatic=cast-makes sense) no weights on the flywheel or conv. = your original crank was forged steel. my book says 2658278 is average 273,318,late 340 crank ,most likely cast=needs external weights. any swift engine builder can easily determine a cast crank from a forged unit by looking at it-no need to even drop the oil pan-look at the parting line rear crank hub. so demonhead now the simplest solution on paper=cast crank 340 damper,good luck finding one-some 72+73 340 only,not the same as 360 but they look similar(340+early 360 counterweighted, later 360 weight removal style=smarter)and a B+M flexplate with a weight for cast crank 340 #10235 and use a converter with no weights.  

let me guess demonhead, either your rebuilder is not a mopar enthusiast and replaced your forged steel crank with a cast unit(not good-out of balance) or is a ripoff and knows the value $ of a 340 steel crank,maybe with a little crack (easily welded and reground) and as drmopar has learned-get the correct steel 340 crank from your average rebuilder=good luck-very rare in 2008 $ or go aftermarket and throw in some lighter pistons,maybe a better damper,Heavy duty flexplate and have it all balanced.  

ALWAYS GET YOUR OLD PARTS BACK, PERIOD!!!!!!!!!!!!!!!!!
I have a 340 cast balancer sitting on the shelf. Anyone interested?