You really want the spacers. There are two for the sides that let the hitch clear the frame welds. They prevent the welds from cracking from the stress of the flat hitch on them. And the two spacers that go under the center portion are needed to get the height & alignment right. Without them the hitch will put stress on the frame when you tighten the bolts. You might even warp the frame or hitch when you tighten them.
So far as fasteners - you want grade 8 for a 6500 lb hitch (they're also used in automotive suspensions). Look for 6 radial lines on the bolt heads. Pretty sure you won't find them at Home Depot or Lowes and will need to order them online. Grainger or Fastenal are good vendors.
The center bolts are the most important, as they support the tongue weight on their threads. Then the rear-most side bolts, and then the front-most side bolts (they undergo shear load when you pull the trailer, and don't have much vertical load to them).
The bolt sizes and torque specs are on the instructions from Sewell Lexus.
Hint: Use some dish soap on the exhaust hanger to lubricate the rubber, so it slides off easier.