I forgot to mention that this original crossmember is also part of the structural integrity of the complete frame. Any alteration or removal needs careful consideration. It is riveted to the frame and adds solid support as the frame flexes. Also, if rivets are to be removed and replaced with bolts, there is a proper way to do this and it is described here on Stovebolt.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."