I dont like the idea of using P clips when you have perfectly usable threaded eyelets. Once setup, you never need to adjust rack and fender. P clips are an ugly hack to be used only where neccessary and they are another nook and cranny where dirt gathers.

To cut a bolt you need a bench vice and a hacksaw and metal file , or a dremel. You have to finish a hacksaw cut with a file to clean up the threads. Always cut on the long side.
Long protruding bolts may interfere with the chain in the smallest sprocket so file back where needed.