Because of amalgamations & takeovers etc. insurance companies are becoming extremely reluctant to take on 'unusual' risks, and an organ van is definitely an unusual risk.
If you are thinking of putting an organ into a vehicle you must do your homework on the vehicle insurance first. The basic problem is vehicle replacement in the event of a complete write off. In my case for example I have a 12 year old Ford Transit Luton van. The bottom book price of such a van is in the region of £350, and this is what I could expect to be paid. It takes only a very small bump or even a reasonable scratch to get to this sort of figure! Take off a £100 excess and I would be left with £250 to find and convert a suitable van - nowhere new the actual cost which I estimate to be nearer £5,000.
Some companies will also refuse to cover you if you carry ANY flammable liquid, for example petrol for a generator, or a butane cylinder for a cooker. Other companies will not take a sign-written vehicle, unless it's a horsebox, when it must say HORSES. Others again will not take a vehicle modified by anyone other than the original manufacturer, or if the vehicle is taken to any event at which the public is charged admission, whether you get paid or not.
So, if you are contemplating fitting, rather than carrying an organ in a vehicle - beware!