The rocker shaft part numbers for a 1970 318 and for a 340-4 bbl are the same. The number for a 340-6 bbl is different. An old rocker shaft that came out of either a 318 or 273 shows wear/gouging at the rockers. It is made of a slightly tougher steel than mild steel, but is not hardened at all.
20 w 50 is pretty close to straight 30 weight. I put together a 318 20 years ago. Crank was fine with over 120,000 miles. Replaced the bearings with federal mogul std. size. The bearings plastigaged at .0015. Ran it for 90,000 plus miles and it still has excellent oil pressure. It is in a 5000 plus pound van, so it worked hard on the hills. It always had 20 w 50 in it most of the year and 10 w 40 for the winter months [CT]. I never rev it when I start it if it is cold outside.