They look like the might just fit under the stock fenders or perhaps stick out no more than what could be covered by a rubber flare/lip. That would save the headache of fender modification.
Another approach if they're just slightly too wide would be to space the fenders away from the side of the trailer just enough to get coverage, then fill the gap between the fender and trailer wall by riveting or welding in a piece of metal to fill the gap. If you used a L-section, it could help support the fender more, too.
Another approach if they're just slightly too wide would be to space the fenders away from the side of the trailer just enough to get coverage, then fill the gap between the fender and trailer wall by riveting or welding in a piece of metal to fill the gap. If you used a L-section, it could help support the fender more, too.