just measure things so its all straight and square to get everything in alignment then they just clamp on with ubolts.
these look to be a great price:
http://www.southwestwheel.com/store/...sion-axle.aspx
just measure trailer frame width so the pads sit under them, you may need to cut off and grind smooth where the spring bolt connections were and then the fenders might need to be redrilled in a new location when your done