Gordon is exactly right on this David. Do as he says, and tighten after driving a bit. The car always settles and/or moves a bit. Either don't cross them at all, or cross only one set of straps. With the front beam, I'd cross the rear only, and not put any side-load on the trailing arms.
I really like the idea of the 2 x 4 on the inside of the tires, that will work perfectly. Leave about a 1/2" or more space on either side, they are a just-in-case rather than a rub-all-the-time thing. My trailer has U-shaped wheel tracks, so it's not going to go too far in any event.
And DON'T leave the car in gear, leave it in neutral, but DO set the parking brake.
Looking good, carry on, David.