I alway put in park if they have a park pawl, or low gear, lock brakes if they have that capability chain so they cannot move side to side or front to rear. If you don't have boomers you can load the tractor, air down the tires, chain it and then air the tires back up but not tight enough to break anything. I see you have a 5020. Big heavy chunk of steel compared to todays tractors. we had a 5010, '68 5020, and '72 5020 until 1984 when we went 4x4. Took cab off 68 , turbo'd it and put 10 ft. deere blade on front. Kept it for a toy until 1992.
Always get a little shook up and mad at myself) when I don't chain one down enough and get to destination and find it walked on trailer. an A is light (though high profile) so you should not have much trouble