I used 2 servo's on my SU76, its a bit rickerty tho - I can't see it surviving my friend's 4 year old's tactic of turning the tank by using the gun barrel as a handle
(you also have to be a bleedin' octopus to assemble the damn thing!)
I did look at industrial ball swivel joints, but I couldn't find a supplier that would ship to 'joe public'