Yes, you will have to open up the pushrod tunnels.
The only correct way to measure pushrod length is just that, measure it. You need an adjustable pushrod. You can make one real easy or buy one.
Set the adjuster at 9/32 inch protrusion from the bottom of the rocker. That should be no more than 1 thread showing. Adjust the adjustable pushrod so there is zero lash. Measure it for over all length and add .050-.100 for lifter preload.
Do I do that with the hydraulic roller lifters Im going to be using?... or something different. First go around with shaft mounted rockers or Mopar. Done plenty of chevys and i would just throw in a solid lifter and go from there.
