I am getting 4:10 gears for my 2001 cobra but wasn't very sure what I exactly needed (parts etc..). My friend is doing the labor work for free! So that's nice. I know the IRS won't hold too much power but I'm going to swap out pretty soon. So what will I need to buy? 31 spline etc..?

 

the cougar i have with the irs in it i went with the 32 spline as well as the half safts i had to have a machine shop to make the bolt on bracets a little bigger do to how thin they were. i didnt want it to have it walk faster than my cougar. it has the same rear as yours but yours might be a little lighter than my cougar. i didnt put in a irs in my gt do to more parts to swap a axel to irs. to be honest a irs deffenetly makes it stick to the road a lot better to be honest. you will notice the difference with the 4.10. i went from a 2.86 to a 3.73 gear it seemed it was more responsive. i have a 4.10 in my gt it deffenetly woke it up

 

oh dont forget to replace the input shaft gasket do to it will leak like a sive do to the different gear ratio

 

i forgot to ask do you have traction controll? because you will have to get a simular posi set up for the 32 spline. if you are going to just have a open diff you will be kind of disapointed. i would keep the posi set up do to helping putting the power to the ground and not swinging the rear out as easy as a posi diff.