you can build step headers 2 1/4 to 2 3/8 but if your running a ex 4inch collector is all you will ever need i think.no need to go bigger you will have a hard time with them if your going under the floor pan. for the primary length of 36 or less but thats hard to do some times. if that trans will not take more then 600 tq i would not want to build to much bottom end