Hot Rods Partial boxing 32 rails?

So I'm putting a ch***is together using some JW rails, model A front and pancaked 40 crossmember for rear and a 6" channel X member. The car will run a 425ci nailhead with a top loader and 9". My question is can I just box in front of and behind the X leaving the center open or will it be too weak for the planned drivetrain?

 

I never box the distance between the X-member on the side rails. I box the front and rear only and have never had any trouble.