Imo buy a 8' long 4x4 (they will clear 95% of the avg 5,000 lb forklifts and pallet jacks.
Also give the operator little leeway so he's less likely to put the forks through the side of the box.
Avg pallet is 48"x48" at the biggest, most are 40x48.
How I'd do it.. 4x4 base, 1/2" plywood, 2x4 across the top perpendicular to 4x4s used minimum 3/8" lag bolts ( like so
http://m.lowes.com/pd/The-Hillman-Group-38-in-x-8-in-Hot-Dipped-Galvanized-Steel-Lag-Bolt/3824909 )
To connect engine stand, 2x4 and 4x4 all together.
I'd use deck screws that use torx bit driver they are alot stronger then drywall screws and they don't loose like nails do. To hold the plywood, and box structure together.. Probably be close to ~100-150.00 in materials.
Sound like over kill.... Coming from a trucker's stand point.. Better/stronger the box easier it is to secure less like it is to fail and damage what ever is in side.
When building/placing the engine avg fork truck forks generally range from 36-48" long, doubling up the 1/2" plywood on the floor would be cheap insurance from an operator not getting far enough under and sending a through the floor,
I like the engine shipping thing... Only thing I could see negative... Is they have niche market, usually that include premium attached. You're also paying for guys that all they do is move engines/transmission s