I’ve said this before, but I’ll be happy to say it again. The neck is the weakest part of the harp and it must be supported. Having a rigid frame for it is the only way I think it is safe to move a harp in this way. Otherwise, you will be needing a new neck sooner rather than later. Why? Because all the bumps and vibrations will travel into the neck, it will twist and torque just a little, and that is not a good thing. Hopefully, you can make or have made such a frame, or get a longer van. Some people lean the harp against the wall, which would allow it to be next to the seats, I think, but it still must be against padding and firmly strapped to the wall with hooks and belts.
Some things are just not meant for multi-tasking, I think.