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I've never heard this engine run...as I said I bought it as a project car and was told the power train came from a 73 duster but never heard it run as it was half apart when I got it...Did 73 340 ever come with a 904? ...this is making me want to just put in a 727 as I can at least get parts that are supposed to work.😒

Whether a 340 came with a 904 is irrelevant to the balance issue. 340's came with a 727 in OEM configuration. The converter was the piece used to balance that end of an external 340. Putting a 727 behind it is not going to solve the balance issue, it's a converter weighting deal (if needed). Trans 904 or 727, in this case, is irrelevant

Can you get it started on a test stand with neutral balance pieces, check for vibration and go from there. Problem is you have no idea how the engine was balanced. It has a neutral balancer on it when you got it. May have been pulled because it vibrated, no way to know without the balance information/card.

Thanks cracked back, The balancer was from a 75 cast crank per the part number so not neutral. That is why I pitched it and bought an aftermarket that was supposed to work on 73 cast crank. I don't have the manual transmission stuff to put it on a run stand. The only reason I want to do a 727 is everything is in a parts book, converter, flex plate etc...shouldnt be a guessing game like I'm trying to figure out now, also sorry I'm not as responsive as I should be, just busy as hell lately... appreciate everyone's help 👍

If you can find a 340 external balance B&M flexplate, that is the way to go if external. The converter can be neutral and no problems swapping in any std balance converter. IIRC the only real difference in the 727 flexplate is it is a little larger in diameter and can be tight with the starter on a 904. I think I turned one down slightly to get it to clear stuff. It wasn't a big deal to get it right.

Funny deal, I run only 904's behind 340-360-408-416's anymore. Never bother with 727's on SB stuff. Unless it's a resto car numbers deal. Hot rod, 904's all day, every day.

Locomotion hit on the key points as well.

All good, life gets in the way....

Using this info you could eiither take a B&M std flexplate or neutral balance converter and replicate the inballance. I may even have the 73 340 chart and weights. I'll look this morning.
Doug