6.3 is difficult to get to.
6.42 is available by putting an ACN/CHE ring / pinion in a 4speed and runing a 1.75 2nd gear. (cut off 5th gear end from pinion shaft)
you can also get a 6.09 in 2nd by running an old Dodge Omni 3.48 r/p 4 speed and put a 1.75 2nd gear in it.
here is my trans chart to help if you need it.