Aftermarket rocker shafts are just as difficult to get through the stands so I know they are NOT undersized.[/quote]
I appreciate your observation, and I totally agree as I have had the same experience. But, in my scenario, with further investigation and using an inside dial-bore gauge, we found that the shaft stand's bore collapses after being torqued down (particularly when over-torqued) & put into service for a period of time ("takes a set"), and does not recover completely (some, yes) when untorqued. This remaining ovality causes an apparent drag, when attempting to remove and/or install them; and therefore may not prove as an accurate measuring instrument to determine the true shaft O.D. dimension.
The most common material Ford used for these stands (FE) was an aluminum alloy which proved acceptable in most applications but was not impervious to failure and certainly doesn't hold size (round). It was (is) not uncommon to find these cracked in the "loop" from being compressed oval beyond the elasticity of the casting, or split at the top thru the bolt hole (usually) or down into the base due to the spreading effect as the stand squashes (& shaft) under the torque load (again effect amplified if over torqued).
Due to the popularity & long production life of the FE in it's many iterations, the rocker shaft assy. (shaft, stands & rockers) were still available thru Ford Parts departments @ the dealers well into the 1980's. By this time Ford no longer produced them, rather I believe they were sourced thru Fred Jones?; who rebuilt these components. We found that the shafts were reground undersize varying somes to "clean-up" the surfaces and put back into service.
In a discussion with a gentleman at Harland & Sharpe (who make rockers for FEs) concerning availability of new ("improved") shafts, he stated that in their search, they were unable to find anyone in the U.S. who could grind a shaft straight & true & to size (affordably), and that there's a great range in sizes (even in a single shaft!) experienced when measuring existing units. And, I concur with these observations.
Remember, we're dealing in .001" or less, plus-or-minus, clearance values here, if you want it even close to right (and that's kinda sloppy, but were not building a space shuttle here).
Also, the cast ductile iron rockers applied directly on the harden steel shafts was not such a good idea ("OK" for stock applications, I suppose). I believe Ford engineering realized such, but maybe the peanut counters won this one, so the quick & cheap solution was to make it a sloppy fit w/ excess clearance and resultant oil wash. Ford used similar components for other applications but w/ bronze bushings, so they weren't unknowing. Scott.