Hi Sally. The "low profile' roof you refer to is on post circa 2010 models. That means the awning rail is approx. 60mm higher off the ground and the roof curve above is flatter.
Is your 320 a pre-2010 model and is the 420 you are considering a pre or post 2010 model?
Not being an owner of a Unico but looking at the pics in the gallery, as you say, you'll have the problem of the Familia length awning cutting across the rear side window of the 420 which might create a drafty seal down the side of the van. Also, most 420s appear to have a locker box under that window although they would be an optional extra.
All in all, I think that it will fit dependent on the height issue, but it won't be as snug as it is on the Familia.
Can anyone else add to this and also give their views on the height issue, if in fact, there is one?
Randa