Chassis Welding - warping

[hotrod56]

Before I get started - wanted opinions. Have a full length chassis to weld up with a center stiffener between the rails. How do you you weld in the center section without causing the rails to pull in towards the center? There is nothing welded to the outside to counter balance the heat. If I keep the rail clamped down, will it stop the pull or will it just spring once its released from being clamped? See the example picture - not my chassis. Thanks in advance.