57 3100 rearend choice

[pete57]

I have a 57 3100 im working on. Ive got my mustang II front from speedway, now I need to get my rearend. Friend of mine has a 79 firebird rear and a 89 iroc Camaro rear both drum breaks. 79 is a little longer than factory 57 and 89 is a little shorter. is one better than the other for transplant? Is there an advantage to running a little longer or shorter? Ill be running 8in vette rallys on all 4 corners and leafspring rear. Thanks..