Ditto what T-H said.
I use 2 x 12 x 8ft. PT timbers. It's heavy but it's clear, no knots, and wide enough to accept the difference between the front and rear wheel widths. I don't drive the tractor up, I push it up backwards. I have a cap on the truck bed and it's kinda low clearance. The seats will usually clear, the steering wheel follows. Just make sure the tires have air in them. You can easily remove the front wheels if you need more clearance.
The tail gate cables are disconnected, the timbers are on top of the bumper, under the tailgate. Yup, the mower deck catches, but the machine is more than half in the truck and lifting the front is a simple thing to do.
I also use a couple old rear tractor tires between the truck bed wall and the back of the machine, to limit the sidewall contact. with fewer broken fenders or scratches. Then it is tied down front-to-back and back-to-front.
When unloading, I have a banking that I can back into, stretch out the planks, the tractors roll right out with no problems. So much so, they don't want to leave 'home'.
Watch out for the hoods. Tie themdown so it won't blow off and get distroyed.