Jag rearends are not particularly strong. A Ford 9" is. Jag rearends are pretty when all the working parts are visible, as on a roadster, but on a 37, it wouldn't be very visible anyways. The frame-----take it to a sandblaster, have it cleaned down to white metal, then assess it. There was a lot of meat on those old frames to begin with, and one bad spot is easily repaired. Boxing it will give all the strength you will require, unless it comes back from the blaster as tissue paper. $4400 will buy a lot of other things that you will need.