Don't know if you have one or not but Summit does sell a device that mimics a rocker arm. You install your adjustable push rod and then put the plastic pushrod lenth checker, or rocker arm thingy on and adjust your pushrod until the checker is flat againt the valve tip and then measure your pushrod. Then order the pushrods you need. I forgot the lenght of mine but they are quit a bit shorter than stock. I also have a roller cam and rockers in my motor.
Remember too that when you do buy your push rods buy about 4 extra, since if you bend one you will have an extra on had right away instead of trying to buy one via mail order.