what is the easiest way to set up the posts for the chassis to sit

As Tex said before, remove the lid of the box, turn truck over, set lid on chassis, move posts around as needed, re-install lid on box, test - repeat as required.

That's probably easiest. I just do it with the box all together because with the 8ight, you have to get the wheel perfectly in the slot... So I put the pegs where I think they'll fit, make them tight enough so they won't move on their own, but you can move them by hand. Then I put my buggy on the box and slide the pegs to where they actually need to be, then tighten them up, and walla.

I align mine be removing the engine so it's easy to spot where the wheel is with respect to the hole in the chassis.

If you do it the way tune says mark your chassis with a dot (sharpie works) as to where your flywheel is orientated within the chassis hole....sometimes flywheels aren't centered in the chassis hole and that should be factored in as you line up the chassis to the starter box.

+1 to that. easiest way