DS2:What armour rating for large truck mounted medium RAM?

I am currently painting up my first Artillery unit (off board I think!).
It
comprises 3 of those large GZG 1/300 trucks (not the semi trailer, but
the
large one with an flat open tray).  On this I have mounted an Irregular
WWI
heavy howitzer (not sure what cal. but it's a biggy). Nice big fat
stubby
looking thing with lost of bits n' pieces, dials, controls all over it.
Looks suitably SciFi anyhow.  I just cut off the end of the base to fit
on
the back of the truck.	Incidently, I could'nt find the product code in
thr
GZG catl site.	My question is, the truck itself is significantly larger
than a lot of my Sig 3 vehicles and therefore I have to class it as a
four.
What armour rating to use though?  Yes, technically it could be defined
as a
softskin but I would argue that due to the load it was carrying, some
extra
plating would have been added here and there.  OTOH, being an open tray
with
this massive gun on top and exposed, there's the argument that for Art
attacks the armour would be 0.	With direct fire against it, maybe I
could
claim a 1 or 2 if people felt sorry for me.  Any advice?

Cheers and thanks
Dave Tan