There is no need to go to chrome shafts because the factory shafts are hard steel.
These engines lasted for years with factory shafts, zero problems. Thermal expansion is not a problem @ 0.003" clearance with both parts expanding together & both have the same expansion rate.
Chrome plating was suggested not for wear resistance, though it significantly enhances that, but because a slightly under-size shaft can be built up to size or built up and ground to size very accurately, and the cost isn't crazy.
Typically for a hydrodynamic plain bearing the desired clearance is about .001" per inch of shaft diameter. In the case of the rocker arms, they work differently than say a crank main bearing because they don't fully rotate and the oil supply is intermittent, not constant. The stock stamped rockers aren't even enclosed and have significantly less bearing surface on the rocker shaft than a 273 type would. How much clearance do you think are on them? They seem to rattle around a lot and it doesn't cause any issues. The spring pressure is going to be the deciding factor IMO. I think the OP could get away with .003" on the suggested build, but it would be a good idea to get feedback from RAU
