recommended oil for '68 383 ?

I doubt that it matters at all. Buy whatever is on sale and change it on a regular basis and you'll be fine. For an engine like that I'd use the cheapest stuff that Costco sells. They usually have a Chevron 5w-30 that is pretty cheap. Throw it in there and change it every 3000 miles and don't worry about it. Save your money for something that matters.
I agree w/ Andy. I ran 20w40 in my "new" 383, 68 form S fastback back in the day. Cant find it now. Also ran whatever oil my sponsers would furnish in the hemi car for 14 years, it didn`t make a piddly dam what brand it was ! The weight ,and being kept clean is the most important part.