Trailers - Towing your mopar?

Check the original center tube wall thickness... 3/8" is THICK. Maybe 3/16"??? Make the added tube no thicker wall than the original.

If I understand what you are saying, yes, the gussets from the next cross-member can 'reach down' to weld to the added tube, since this added should only be taking vertical loads. The diagonals up front should take the side loads on the tongue.