generally speaking, and dont hold me to this as some of the more recent vans have a raised section below the seat that isnt there on my mk2 ...... to replace seats is not so dificult, if you are lucky maybe 2 of the bolts will line up, if your not then you need to remove all fittings from the floor, seat runners, mats etc, position your new seat where you want it, mark the foor, then look around and see if anything is going to cause you a problem, like to close to original holes or how it fits in relation to the ribs in the floor etc
then get underneath and work out excatly where your new holes are going to be, and make sure there is nothing to get in the way, like cross strengthening plates or original seat area strengtheners .... while your under there see what ford used as a strengthenning panel as you will need to make something up to go under your new mounting position
then when all is checked, drill new holes, make up your bracing pieces for underneath, bolt it all together and then spot weld the bracing plates in a few places, clean the area, paint it and slap your mats back down, and then put some silicone sealant down the holes before you screw the seats in as this will seal it up and make sure it doesnt leak water in, then once finished get underneath and slap some underseal around the bolts to make sure they dont go rusty
thats it really, nice and simple
the far more complicated and not really worth it method is to make new low profile frames that bolt to your van and the seat then bolts to them ..... think stud adapters for wheels, that kind of idea
or if possible modify the seat to match the van, i think its just easier to make new mounting positions in the van myself
