JB Weld

DocCreer... I have a near MINT AIR tube for a 1982 AMC Concord for sale. I only used it for one year just to pass emissions testing one last time. I'm not sure if it will be the same as your Eagle... but I have the rest of the emissions stuff too.... it will never go back on my car... 

And as far as cracking, that was a bigger issue with the early style. The later ones, in my personal experience with Eagle, don't crack often. It's a matter of was it put on correctly and the torque specs followed for the most part. 

If you have put on the replacements, then you also know they are not always flat, and sometimes you struggle to get the end fasteners to line up. 

Face it, it's chinese cast and specs. 

Now most folks know I don't generically diss Chinese products, some are actually superior to USA made due to economics - 

I can point to specifics, yes, but in this case, i've dealt with AMC 6's for over 40 years.  I have NEVER had a manifold on any of my own crack, let alone leak. When people have brought them in for repairs, I've gotten them resealed without issue - and I follow the TSM specs to the letter most of the time.

I bet even on the hundreds of customer cars, I've only replaced a couple due to cracking - and those were the early style, bolted to the intake. Those had more issues than the later style - in MY experiences.

One other reason I commented as I did is that I also know the OP doesn't have ready cash on hand all the time.
The fellow tries but is on a tight budget unless things have changed for him in recent years. He's a willing and eager learner, though. 

If it was a manifold of mine I was selling him, it would have the studs replaced - that's a no-brainer and they are actually more easy on the 6s than the smaller studs used by other companies. It's also not that bad to remove them if you have patience. Really, a few bucks vs. 120 - the studs just won't enter into it if it were me choosing. What's a few minutes vs 100 bucks??

On the other hand, if it was a manifold off my shelf - why would they need to replace the studs? Does everyone replace them, need it or not? The ones I have have studs in them I'd likely not mess with because they are fine. After-all - I removed the nuts in a muddy junk yard setting with common hand tools, so I know the nuts weren't rusted onto the studs but came off pretty much ok. 
Funny, everyone is asking for trouble where there is none.

No stud problem - my spares have good studs, but I'd happily replace them of a buyer asked.

Who says these would crack or wouldn't seal - IF put on correctly? I've not lost one yet and have had AMC 6s since the early 70s - 232, 258, etc. at least a half dozen of my own, and the first shop I worked in was a former AMC dealership shop - can't recall replacing exhaust manifolds, but with the pre-81 systems we DID free up a lot of the heat riser valves! (and replaced a fair number of burned out choke heater tubes in the 232s, etc.)

Me, I'd opt for a good used manifold for a few bucks - because I've never had trouble, as opposed to spending a lot on one that also could crack!
Who is to say that the NEW one wouldn't crack if a used one would? Did China make it better for that price?
I know it's a calculated risk, but the owner has to determine his budget vs. his TIME and willingness to work a little bit. If a used one cracks, you get another for another twenty bucks and are still 80 ahead.

If the new one cracks, you remove it and get it replaced, but at what cost for shipping, etc.

It's not that bad to remove the studs - IF it's even needed, because most I see are perfectly fine, and if it was a good AMC person selling him a manifold, that AMC person selling would ensure the studs were good or replace them. I sure would if I were selling. 

They don't crack that often, and there's no saying the new one wouldn't. A used one has survived how many decades and will suddenly crack? Remember, these are the better 81+ manifolds, not the early ones that bolted to the intake - and those did have more issues. 

Sorry, I stand by my experience and comments. For someone on a budget, I'd easily go good used because the arguments against just aren't that solid. Especially for an owner on a budget. 
If I have more than one spare on the shelf, I may even consider selling one, with either good used studs or new. I'd have to look at my stash. 

He's got his information and can choose with path he wants to go based on all sorts of information and experience. He needs to have both sides and both experiences to make a decision best for HIM, not for us. 
You may use new, I would absolutely use used - what he does is up to him.  

What you do is up to you and isn't right or wrong.