Taking into consideration that your 460 Big Block Ford is tight, and runs normal, you can use two viscosities of oil. Choose the one which you feel is best for your situation, I am thinking of a name brand, dino oil, API SL or SM certified lubricant:

10w30: Lighter weight oil, pumps well to top of engine at cold start-up. Flows much better during cold start-up and through normal engine warm-up cycle. Lighter viscosity frees up internal friction at crankshaft and bottom moving parts of your 460. Allows engine to turn RPMs easier. Your gasoline mileage will be the best possible that your 460 can deliver, in whatever condition and state of a tuneup it's in.

20w50: Thicker oil, pumps slower when cold, to top part of the 460. At start-up and through the normal warm-up cycle. Higher viscosity adds more internal friction. Starter must work harder to turn over the thicker oil when cold as compared to a 10w30. Causes engine to turn rpms harder and slower. Gasoline mileage will drop, delivering less than top MPG the 460 can deliver. Less miles traveled per tankfull, more stops for gasoline at $3 per gallon, + or - $.30 cents...

Your choice. It's all there. Both oils will lubricate moving internal parts in the 460//. Hope that helps.