Unfortunately, it appears to be REAL for a number of reasons:
1) It would be impossible to fake it or the actors would have faken it differently.
If the aim was to entertain the cloud, it would be prepared much more intensely and dramatically. There is not enough blood simply because she was lying in a box that caught the majority of blood. If this was an intended performance, they would sprayed the blood everywhere and not bothered spilling just a little bit on the floor so that barely anyone could see it.
2) Her movement coordination. It would be extremely hard or even impossible to coordinate the exact movement of the hand when there was an alive person because she wouldn't see the exact position of the saw. As well as spill the right amount of blood out of the head's mouth. As you can see, it follows exactly the moment when the saw hit the neck.
3) Real blood. The blood seems chunky, dark and thick, exactly what it would look like in real life. When it was a prank, they would've probably use some sort of red liquid or ketchup, which has much less denser consistency and would make more mess, just for the effect.
4) Media coverage. If they faked it, there would be a purpose (exposure, commercial, TV shows...) and it would be revealed, because that's why they've done that. There was no revelation, which tells me somebody wanted that material to disappear and then some sort of a prank channel took it over, thinking it was fake. But if you look at all the pranks, there is at least some sort of evidence of the purpose they were made for. At least the NAMES of the actors. What would be the point of investing tens of hours into rehearsing it and then keeping the point to yourself? Doesn't make sense at all. Seems to me that they wanted to forget that.
5) Their reaction. What would you do when you planned a trick like this? Would you say "and at the end, everyone will just stand there staring at the scene"? No, you've probably prepared some dramatic reaction. This tells me that this is totally genuine, because this is how people react in a shock, not how people think they would react.
6) The metal plate I think is no evidence, because in normal tricks they are using it too, exactly the way it was used here. So, probably made of some flexible material.
7) He didn't seem to have cut the head off completely since he probably realized what he did when blood started pouring out on the saw.
8) Purpose. In the 60's, the only reason of doing this kind of prank would be to entertain the audience, nothing less, nothing more. There weren't any viral videos at that time, so I assume they would do anything to make it real on stage, not for the camera. They are actually doing the opposite, there is just not enough drama. There are examples like this when performers wanted to make it interesting for the audience and it's full of blood, shouting, panic. This is the opposite.
Overall, the movement, the reactions, the blood spill, everything seems too real and too well elaborated for just one night. If this was a prank, that would be an extremely hard to do and bad looking prank. When there is a shootout, the majority of people is thinking there is a firework. Real tragedies do not look like in the movies. They look like this.