I personally wouldnt rethread a AC556 and/or run it suppressed.
The AC was never designed to be run with a suppressor and you will be putting more gas back into the bolt/carrier and it will slam home harder into the back of the receiver. The AC has no buffer in the back so it metal to metal contact as the bolt and slide come back. If you put a suppressor on a AC and you will be battering the bolt back into the receiver harder than it was designed for and potentially develop a crack in the receiver.
You will also be modifying a factory machinegun from its stock form. If I was buying a second AC I would want one in its factory original configuration with the original ruger AC flash hider vs. an aftermarket A2.
If somebody had cut and rethreaded the barrel, I would be concerned on the wear/tear on the receiver since the previous owner ran it suppressed and I couldnt return it back to the way an AC should look with the distinctive Ruger flash hider. All else being equal I would offer less money for a rethreaded AC than a factory correct version, especially if it was a factory 13" model as those barrels are difficult to come by.
That said, its your gun and unless you plan to sell it you can certainly do what makes the most sense for your use case. (there may be other buyers out there that would pay a premium for a rethreaded AC as well)