Won't matter what they thought back then (Ford that was)
1. Warranty is over, a lot of their recommendations are "CYA" driven
2. Today's oils are not the same as oils from then. Different additives and such than were available then.
3. Fresh rebuild, tolerances may be different from new, (everybody's idea of a "rebuild" isn't the same) and today's rings, piston materials, bearings may be different from brand new.
4. How YOU drive, how much you let it idle, vs anyone else, how often do you change it, also can have an effect on exactly what weight to use.
To me they spec out way too thin of an oil now a days. Being a fresh engine, I don't think you need 15w40 or 20w50, but it's not an OHC engine with valve timing actuators either. (Like today's engines)
On a 60s-70s-80s straight 6, in somewhat normal driving, I'd go 10w30 or 10w40 conventional.
I've seen 20w40 listed in old manuals, but never actually seen it for sale anywhere.