well, I have no experience with chevy/Ford/Magnum heads but here is what I found:
"...Finding Center There are several ways to find proper pushrod length, but when it comes to stud-mounted rocker systems, the simplest way is to use a Sharpie and a wrench to turn the crankshaft. Even the most sophisticated engine builders use this method. Keith Dorton of Automotive Specialists says it is the most reliable and simplest way to determine correct pushrod length..
...you can use an adjustable-length checking pushrod designed especially for this task....
Whether you are using a checking pushrod or a standard piece that you think might fit correctly, begin by coloring the valve tip with either a Sharpie or machinist’s dye. Then, install the pushrod and rocker arm while the lifter is on the base circle of the cam. If you are using a solid lifter, set the valvetrain to zero lash. If you are running a hydraulic lifter, tighten down the rocker adjuster to your normal preload. Using a wrench on the nose of the crank, turn the motor over several times, then remove the rocker and check the mark left on the valve tip. The roller tip on the rocker should have left a shiny spot where it wore away the ink you placed on the valve tip.
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If the pushrod length is correct, this mark should be centered across the top of the valve stem. If it is too high (closer to the lifter valley), try a slightly longer pushrod or lengthen the checker. If it is too low (closer to the exhaust ports), try a slightly shorter pushrod. Now simply repeat the process, adjusting the pushrod length each time, until you have the wear mark centered on the valve tip. Sometimes the low-tech methods really are the best."
So I guess you still gotta spend some dough to run "correct"
