I've thought it through too, and come to the conclusion that cutting them would be a bad idea. They will have been drilled to the right depth for the studs on the car and the studs on the adapter, and cutting either side would reduce the amount of meat each one is using, leading to almost certain failure.
I will have to keep looking.
I'm a step closer to being able to measure up properly now though. I bought a used tyre in the size I need (£11 inc postage), so I can mount that on one of the wheels for trial fitting. I can lay the springs in place on the the axle and measure those. Front end is going to take a bit more work, as I will need to rebuild one corner. That said, the arms are still on the drivers side, so I only need to fit the spindle and brakes...