As others have said, you NEED a handhole box for this application, a floor box is the wrong gadget for the job. Ask you supplier for a "Christy Box" and see what they have.
Check out their catalog: http://www.christyconcrete.com/elec_com_boxes.html
You probably want an N9 or FL8. Set it in a base of crushed rock so the bottom can drain, bring your pipe in the sides or bottom, and pour your finsihed concrete around the sides. Use waterproof wirenuts if you are splicing in this (though it's better if you don't), and provide bushings on all your conduit.