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The aluminum supports allow some deflection at the end bolts for the rocker shafts which over the long haul could allow the shafts to break at that point allowing the end rocker to fall off. You'll know it immediately by a popping sound back through the carburetor. Although the aluminum supports are more prone to breaking shafts when running higher than stock valve spring pressures, I've seen a several 360 and 390 pickups in the seventies and eighties with broken shafts due to the aluminum shaft supports. Age fatigue on the shafts more than likely contributes to breakage problems that are encountered while running stock valve spring pressures although the rocker bolts not being properly torqued can also contribute to the shaft breakage. Those thick washers that came from the factory under the heads of the rocker shaft bolts are also mandatory as they provide additional support or pressure to the top of the rocker stand.

The factory iron supports or aftermarket steel supports make the shafts more rigid thus cutting out more of the flex at the end rocker support bolts which will prolong the life of the shafts. The 406's and 427's came with the hefty iron rocker supports and still show up occaisionally as used parts for sale. There were two different widths available but that's easily worked around also. Although the Hi-Riser engines had a shorter support, they do not show up too often. Easily recognized if they do as they are considerably shorter than all the standard height units.

If you know someone in a machine shop, steel supports are quite easy to produce. Doing it yourself out of the appropriate width barstock could be performed with as little as drill press, bandsaw, dial calipers, and a reamer. The measurement from the bottom of the shaft to the hole would have to be maintained to a pretty close tolerance though to keep any additional flex out of the shaft when bolting them down.

Although expensive, end supported rocker shaft assemblies will cure the problem with all valve spring pressures.