what we did is took some wooden dowl rod same diameter and made a few peices the length of the stockers and then in increments that are commonly available and checked em.
stock length works fine for the combo im referring to, i doubt that the P heads are different...altho the heads we used were 94 Cobra GT-40 with FRRP 1:72 it was just fine.
maybe some else can back me up.
i could have saved you some dough on that cam tho...still looking for a buyer hehe
Choose one cyl to check the length. Get a black marker, and color in the top of the valve your working on. Put the rocker on, and set it correctly, Rotate the motor over a few times, then remove the rocker. Look to see where the contact patch is on the top of the valve. If its low on the valve(towards the headers), this means the PR's are too long, but since you have pedistal heads you can just use shims. If the contact patch is too high on the valve, then you will need longer PR's. Using an adjustable pushrod is the way to find the exact size you need. You want the contact patch right in the middle of the valve.