I've had a look for you, as I bought a Mk8 with a single passenger seat.
My seat base looks the same as a Custom seat base - when I look at one on EBay here...
https://www.ebay.co.uk/itm/Ford-Transit ... :rk:1:pf:0 The base on my Mk8 has a bolt on each corner holding it to the floor. These are 340 mm apart (front to back), and 400 mm apart left to right (measuring in there as best I can as I fitted swivels).
Could well be a simple bolt-in job. Makes sense that Ford would use the same seat base for the Custom and Mk8 single passenger, and likely all Mk8s are made with holes for both single and double seat bases.
IMG_3125.JPG
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