I don't believe there is any evidence to suggest that neurons elsewhere in the body have anything to do with mentality--lots of folks have had gastrointestinal problems where they've ended up with parts of their gastrointestinal system damaged or removed and there's no evidence that that's affected anything mental in those individuals.

Beyond that, I don't know why people so readily buy the core ideas of functionalism, but unfortunately it's almost considered gauche to question those assumptions.

The "enteric brain" operates independently of the other brain mounted in the skull. This makes evolutionary sense, given that our earliest multi-celled ancestors were, essentially, all gut. Besides, as one physiologist noted, digestion is very complicated and our 'higher' brain, especially, doesn't want to be notified of every digestive detail.

Are the octopus's neurons located in its far flung tentacles not part of its mind? I'm no anatomist, but maybe the distributed brain of the octopus is necessary. If it is going to match it's skin to background, the centralized eye has to identify the background, and then send information to the part of the octopus that needs to change its spots.

IF one could upload an octopus mind would it not be as much an octopus mind as an uploaded human mind would be a human mind? What would our uploaded minds be? My guess is that both octopi and humans would prefer to experience their minds in their natural environment.

-lots of folks have had gastrointestinal problems where they've ended up with parts of their gastrointestinal system damaged or removed and there's no evidence that that's affected anything mental in those individuals. — Terrapin Station

Anecdotally, I heard otherwise on an episode of RadioLab, where a guy had to be fed from a device because of a hole in his intestines which the doctors couldn't surgically repair. He developed the most intense cravings for food that nearly drove him crazy. At one point, he ended up in some stranger's backyard, taking over their grill, just because of the smell of the food.

http://www.radiolab.org/story/197243-gutless/

Also, there's evidence that the bacteria in your gut influence your mood.

https://www.nytimes.com/2015/06/28/magazine/can-the-bacteria-in-your-gut-explain-your-mood.html?_r=0

Anecdotally, I heard otherwise on an episode of RadioLab, where a guy had to be fed from a device because of a hole in his intestines which the doctors couldn't surgically repair. He developed the most intense cravings for food that nearly drove him crazy. At one point, he ended up in some stranger's backyard, taking over their grill, just because of the smell of the food. — Marchesk

Yeah, he had a hole in his intestines. That's not evidence of his intestines amounting to mental content, haha.

Also, there's evidence that the bacteria in your gut influence your mood. — Marchesk

And neither is that. The idea is that it changes the chemical balance in your body, including your brain.

...their digital clone can live happily ever after, freed from their meaty existence. — Marchesk

It would have to be more than just a scan of a brain; a fully working model that 'processes'. And then, if it isn't going to model total sensory deprivation, which I don't think would be a 'happy' state, it would need some complex input that would model a body and an environment on an ongoing basis moving forward.

It wouldn't have to be exactly the same as the meat, but it would have to have some considerable similarities to provide input to all or at least most of the sensitivities. Something like a complexly sensing avatar in a virtual world.

One can draw a line round the brain as processor and model just that, but it cannot 'live' without a world of input, and the input has to make sense,which for sure means modelling the arms and legs too and an environment in which they can operate. Unless happiness is total paralysis and total sensory deprivation.