I see nothing wrong with boxing the frame as a rectangle is significantly stiffer than a "c" shape. You will need to X-brace where the original Trans crossmember was to keep the front suspension rear spring perches happy.

A doubling fish plate is a good idea where the new crossmember will be added.

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me