more frame advice

hi all, i am building the frame for a 1928 tudor sedan from 4"x2" 3/16 wall tube but can't decide how wide to make the rear cross member my plan says 35" but if i make it 45" i could use the shock and spring mounts that a factory on the 8.8 rear end,it seems that 45" would fit inside the body. looking for advice and what widths others have used, i also plan to channel the body 4", this seems to be a major hurttle for me since this is my first time doing this, and i really want to do right. thanks for any input