http://www.autohobbydigest.com/a-auto.html

ALL 904s are short, ALL 727s are long...starting in 67 all A-Body autos got bigger trans tunnels and 727s....

there is also some info there on crank and torque converter intercahnge...they chnged TC and had to chnge the cranks to match...important info if you are crossing the 67/68 year 'boundary' with either item...