I have used two custom folded 4mm steel 'C' channels to sit inside the original chassis rails. Using these as well as lengths of 30x30mm SHS in the gutters I lined up the two pieces. I then checked various points with stringlines and made some minor adjustments to a point where I think (note think not know!!) it is now pretty straight. It's hard as there aren't many straight sections in/on the van as most things have a slight curve so stringlines are pretty hard to set up. Measurements inside aren't much better as one side isn't always the same as the other making it hard to get diagonal measurements inside either. It looks pretty spot on when sighting down the gutter and along the outside of the van. Below are some pictures of the two pieces being 'joined' together.
I did not join on the reinforcing member but slightly offset for the walls and roof and joined the floor on the cross member. For the window on the far side I joined the inner and outer skins in two seperate locations to add more strength and also allowed me to gauge whether everything was lined up properly when the outer and inner window skins matched properly.
Hopefully get the back section joined to the front two over the next couple of weekends. So keep checking back! :-)
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