I always use FedEx, pack the engine in a large rubbermaid tub. I've done this at least 6 times. Used to use DHL, but they no longer ship domestically.
Lay a sheet of 1/4" plywood in the bottom, then thick, very heavy solid foam, about 1" thick is all you'll be able to squeeze in.
Set the engine upside down, sitting on the head & cradle top. Pack the sides with solid foam, then lay another sheet of 1/4" plywood on top.
Pop the lid on, then drill the edge every 8 or 10" all the way around with a 1/4" drill bit. Zip-tie the lid down then tape it with duct tape.