good point, yes the shims are very important, be sure to use the adjuster/measurement rod that comes with them (I believe I had to buy a roller rocker shim kit from summit for $8 now that memory serves me. With that, you'll be able to adjust as needed to allow for the stock rods. You'll know if it won't work with the shims, then that will tell you you'll need a longer/shorter pushrod (unlikely though if your motor has never been rebuilt b4)