1. use a pressure treated 4x6 block under the trailer jack so you'll have plenty of clearance, you don't want to do things twice. also cut 4 other 4x6 wedges to block the trailer wheels. 2. Do not use 2x8 pine Homedepot stuff, their strenght is on a vertical position not horisontal (flat) use instead scafolding planks they come on a 2x10 or 2x12 they are meant for horizontal strenght, the longer you get them the less pitch you will have and easier load and unload, for the trailer you are describing and front facia on your Z I would say an 8' lenght with a center block under each for support should do it. you would not find these at Homedepot. Go to either a lumberyard or a local scafolding company.