I have an original station wagon here so if you need photos of the chassis mods, let me know and I'll see what I can do. They are minimal though really, remove the front 109 utility floor supports and fit lower ones for the middle floor. There is then a support for the front of the tub and some brackets on the outrigger for the tub to bolt too, that is about it. Oh, Grind the tailgate hinges off the rear crossmember if you want true station wagon appearance.
I may be overly cautious here, but unless anyone has specific information to the contrary I personally would be concerned that this would be regarded as major modification.
It doesn't sound like much, and it really isn't, but there is a difference between an interpretive view of the term major modification and the technical definition of the term as applied when assessing a chassis, ie something can constitute 'major modification' in an official context which from a general point of view you would not think would do so.
The discussions I have seen on other forums around this point (mainly MG related) have reached the conclusion that cutting -anything- off the chassis whether structural or not constitutes major modification in an official sense, whereas adding things within the existing chassis envelope does not. For example, if you install a different engine which needs different chassis mounts, cutting the old ones off and moving them would constitute a major modification whereas simply adding additional mounts would not. Cutting a crossmember and welding it back in a slightly different place, or fixing it with bolts instead, would constitute a major modification but welding extra tabs on to the chassis for mounting points, e.g. seatbelts, would not.
The mods here are mostly additions but there is some subtraction so it is right on the line and I wouldn't care to call which way it would be interpreted if it was ever inspected. In principle, it is still a factory specification, but not for the same vehicle model. This is identifiable from the vehicle ID prefix.
In practice, I think the change is subtle enough not to get picked up but there is a difference between being certain you are the right side of the line and not being caught on the wrong side. Personally, I think this is too close to call as to which side of the line you would be, and I wouldn't want to put all the time, effort and money into the vehicle with any uncertainty involved.
The VRO can advise better than me what the current state of play on this point is.
Alec