To shim or not to shim

Ok, so at least I caught this early. I do have a set of adjustable rockers from a 273 or 340, will they work? On the push rods, how do I know which new set to buy, or even if I need them? What did you do to find out the hard way Jack? Maybe I should just stop with the Magnum heads......thoughts!

Used to have an uncle that lived in Brandon, I was from just south of Meridian,,,,,Was.