You won't have major probs finding a swivel to fit. As said, bracket/spacer welding may be needed. Sme diyers have used refabricated brake drums etc. and made their own.
The swivel will raise the overall hieght a little which should'nt be a problem. You'll just have to examine to see if there is space for the seat base to spin freely. Some years back I considered it, but felt it would'nt be practical/worthwhile to try. I think most owners get proficient at the (back rest down) reverse somersault method of getting in the rear
....and if the seats are forward you can sit on the bulkhead to face the rear
Here's a vid for interest
https://www.youtube.com/watch?v=fa0fWBVdZJU
and the 'sbmcc' website used to have a few articles/threads
Do take your van to a motorhome supplies if you need a better opinion...talk to a fitter...this is where you would buy the swivel anyway
Here's the link mentioned in the youtube vid
http://www.wyvernleisure.co.uk/advice_and_information.php?&dx=1&ob=3&rpn=parts_accessories_and_services