I'm no expert - I've only been in this hobby a bit under 2 years (no casualties so far probably thanks to invincible fish). This is my 2 cents worth..
Logic would say that if you've known him for 8 years you'd know what his normal behaviour is and you'd be well aware if he was stressed.
As it is, you say he seemed happy enough in the 34, so he probably is... I'd keep him and save him the stress of having to get used to a new tank owner and housemates.
I know it's against the 'rules' to have him in that size tank, and in no way would I suggest going out and getting one for a tank of that size, but you've not been irresponsible, and you've ended up in the situation anyway. You just have to make the best call you can.
As the saying goes: "Rules are for the obedience of fools and the guidance of wise men."
I'm still trying to work out which heading I come under..
