Yes, the late '30's Chevy frames are already boxed from the factory. For those who have never looked at these, the frame channel is an upside-down U with a lip on both edges. Then a flat "boxing plate" is riveted and spot-welded across the bottom of the channel. Back before the advent of fabricated box-tube frames with independent front suspension, the east coast Modified guys really liked these old GM frames. They were lighter than the '40 - '48 Ford frame, had no X-member in the way, but are quite rigid. The hot setup was a '36 -'39 Chevy or Pontiac frame with '47 - '48 Ford front axle, spring, & split 'bones, big Olds, Pontiac, or 9" Ford rear on parallel leaf springs and the coupe body of your choice. Stuff in a healthy V-8 from your favorite manufacturer and go racing...

If your frame is in reasonably good shape, it should require no additional boxing.