I fitted a Mk3 power box to a Mk1 camper about 8 years ago. Now if I remember right-
The mounting holes are different so I made a large right angled adaptor plate out of 6mm steel plate, cut out the existing mount and welded it in it's place.
I used the Mk3 drop arm but the front ball joint on the drag link had to be changed ( take drop arm & link to a motor factor and annoy them).
My Mk1 was a diesel so standard Di pump then the hoses where fabricated at our local hydraulic shop.
The column was Mk3 but with Mk1 steering wheel and switches grafted on. Can't remember details although some thing to do with a bearing at the top.
I recall studying an LDV400 box and nearly going for it as there were loads around then but a Mk3 one come up.
Hope this helps
Plodding along.