As for oil weight...
Check the shop manual, but I think that 10w30 is recommeneded for a 1960 235. Dunno about the SBC, but I'd assume it's similar.
I used Rotellat T 15w40 in my 216 when I changed it last week, and while I think it may be a bit thin @ 160° - 180°, it's got a thicker top number, so it should be darn close to 20 or 30 once the engine warms up.
I'm thinking about ordering a case of SAE 20 (They make a Rotella in that grade) and giving it a try in the spring.
