will this shock work

[rmuscle]

if i have a set up that calls for a rear shock of 13" would a 12 3/4 shock work

[tntauto1127]

It will work. It will just make your seat set 1/4" lower and maybe give you 1/4" less travel depending on the shock. The way the shock is valved will determine how well the shock works.

[wrhracing]

As far as fitting in there yes it will work. More importantly is the amount of shaft travel available. If the shock is 12.75 " because of added spacers for length/height then that may not be the shock you want.
Check bushing sizes and bolt hole sizes, but definatley look at the available amount of shaft travel.