i've seen a mk1 van with bubble arches and yes it did look shit. i reckon it would look best with standard rear arches and mexico front arches...
but i don't think the rears will quite fit with 7.5" wide although they may squeeze under on an anglia axle or specially built ultra narrow atlas about 46" depending on wheel offset and tyre size.
you could 'tub' the arches inboard if you really had to have the wide wheels
i fitted 6x16" wheels on a standard arch mk1 lowered 3"

don't want to be pessimistic but i doubt you could get a DIY cossie conversion done for that money