Brian,
You are correct about the style-change in the mechanical lifters:
Shop Manual reference 1 Note: the "EARLY DESIGN" in the photo above refers to 1958, the first year that there were oil passages in the block and head, to provide oil to the valve mechanism.
Prior to 1958, "Lubrication to the valve mechanism is supplied by a drilled passage from the rear camshaft bearing to a pipe located under the push rod cover to a fitting at the top of the cylinder block. The oil then flows through a passage drilled in the cylinder block and head to the valve mechanism oil connector pipe between the two hollow rocker arm shafts, then distributed to all rocker arm bearings. A bleeder hole in each rocker arm supplies oil for lubrication of the valve stems and push rod sockets."
Shop Manual reference 2Maybe someone with a Master Parts Book that covers through at least 1958 will post the length for the push rods.
