400"
Your certain it's got a cast crank? I know 99.9% of 400's had cast cranks, the exception being some 4 spd passenger cars in 72-73... But also a 383 crankshaft will drop right in & they are almost all forged.... If you've visually inspected the crank it's pretty easy to tell...
You've tried adding weight, by your description you are dealing with a balance issue.... Shooting in the dark, I'd try going to a neutral balance & see what it does....
The vibration is present even in park when reving the engine ??? The trusted speed shop would almost certainly know if it's internally/externally balanced or not...Last few engines I had balanced wanted everything.. As in balancer,flex plate and converter..
If you run the engine up to the RPM of vibration, then throw it in neutral and it still vibrates it is your engine. If not, it is your converter back.
Hmm: I'm not so sure I would start grinding weights off. Double check your ignition firing order first, pull all the spark plugs second and have a look for unusual colouring. Then I would pull the valve covers and look for a bent pushrod or anything else unusual. After that I would try to find a neutral balance convertor and harmonic balancer and bolt them on and try it.
Then, sad to say I think it would be time to pull the engine out and have it balanced.
You say you never ran the engine before the rebuild. What did it come out of? Not that it really matters but it sure sounds like miss matched parts if you say it runs good. Been there before!
I hate to say it but it looks like you may have to pull that baby apart and have everything check for balance and potentially balance it. If material has been cut away then you have no idea what the last guy was trying to do with it before you.
This is why I love the kits from 440source because everything is already balanced for you. Heck I even had my machinist send it out to check for balance and yep absolutely nothing had to be done.
Definitely a cast crank & as you observed someone has been taking metal out of the counterweights which makes no sense... Hate to say it but I think HP Cuda is correct.... That sucks...
That two different rpm range vibrations are a classic indication of imbalance
Seeing your picture with the pan off I see another possibility... Looks like an aftermarket flex plate.... Is it the one with a chunk taken out for balancing with a cast crank? If so are you also running the convertor with the weight for a cast crank? Cause it's one or the other not both...
FWIW I looked for pictures of a 400 Mopar cast crank... Got allot of Chevy cast crank photos & Mopar's with strokers but I didn't see two pages of Mopar cast cranks....
Just went back & re-read your post...
"
I thought it was the converter at first, since the engine is externally balanced, and it had the small 10" converter. So I changed the flex plate to a b&m flex plate, and a new stock replacement converter. Still the same vibrations. "
So I'm thinking the first vibration was a stock flex plate & a convertor without a weight... Second version was weighted convertor & the flex plate that doesn't need a weight.....
I see one balance hole in the crank online, I see at least three in yours.... Thing is cast cranks are lighter than forged cranks so in order to get things to balance they drill a big hole through the throw so the counterweight can weight less forged cranks don't get the throw drilled cause the iron is so dense the counterweights need to be drilled to reduce their weight..... Cast cranks usually get drilled in order to add mallory metal which is super heavy.... If someone took allot of weight off either they had a light rotating assembly or they wanted extra weight on the damper & convertor...
What brand is the convertor and did you take a photo of it by chance?
These engines are pretty old and you don't know where they've been so I have learned to always balance them, your situation is why unfortunately. Sorry about the trouble.