The interior tub (or floorpan w/ firewall & rear package shelf) can be placed inside the body, then the chassis will rest on that. That's how the kit will assemble in the end, and will allow you to see where you want your wheels in the wheel opening.
I use stacks of pennies front and rear, under the chassis, to shim the car to the stance I want on my workbench, then figure out how to make it happen with the suspension pcs, etc.
HTH.