I am building some simple “pallets” out of 4x4 lumber to ship some machines.

The plan is to screw the feet of the machines to the top of 8 foot long 4x4s, and then to screw shorter 4x4s, probably about 4 feet, to the top of them as crossbars to move the machines with pallet jacks. Basically, I’d have a rectangle 8 feet long by 4 feet wide with the weight being supported from above by the crossbars. I’m not sure how much the machines weigh, but as a guesstimate, I’d put them at about 600 lbs each.

My question is what type and length of screws should I be using for this? I may double up on the cross bars if it doesn’t feel strong enough to safely carry the weight. Given I’m using 4x4s, I’ll have about 7" of wood to work with, I imagine 6 inches would be a minimum length, but I don’t know if I need some thicker screws, in which case I might need to drill some pilot holes.

Thanks for the help