I fitted a fasp seat swivel to my drivers and passenger seat last night, they are fairly straightforward to fit apart from you need some machine bolts with flat heads to bolt the swivel to the seat base otherwise the heads catch when you swivel the seat.
The drivers seat seems a little high with the swivel fitted as my knees are now about an inch from the steering wheel, this is because it lifts the seat by about an inch. The Atlantis seats I’ve fitted are very firm as they’ve not been used much so with an older saggy seat this wouldn’t be such a problem. I now also have to lean forward to reach the gear knob when changing in or from 3rd or 5th. So I fitted a momo gear knob as far up the lever as possible and this seems to have helped, I’ll drive it a bit more and see how I get on if it becomes a problem I’ll buy an extension.
One way around the height problem would be to lower bases which are available, I had one on my last T4. However I intend to fit my leisure batteries under the front seats and if I fit lower bases this would restict there size even futher.