HMmmm.. consider that the rocker changes the direction of the pushrod, if you lengthen the valve stem you'll need a shorter pushrod, if you have a shorter valve stem, you'll need a longer pushrod. There is a certain length "range" that you need to stay within so you can effectively adjust the lash of the valve train.
The machine shop (I would think) can give you the original measurement and since it's a chevy, I'm certain the tech guys at Summit or Jegs know it by heart / committed to memory! :LOL:
Hope this helps!