If your engine was rebuilt in the 70's, there no need to worry about oil now. The cam is well broke in and there is no need for the extra zinc with a stock camshaft. If you are worried about wear, use half a bottle of Comp Cams break in lube each oil change. You can also purchase Valvoline VR1 race oil with zinc for around $5 a quart, with that you need no other additive. You could get away with half race oil, half standard 30 weight and still have plenty of protection. Diesel oil has reduced the amount of zinc, just not as far as regular oil. All diesel now run converters on the exhaust which started the reduction 4 or 5 years ago.
As far as running with Rotella or other diesel oil, you won't notice a thing different other then the smell. Rotella smells funny, at least it did when I ran some. Diesel oil uses the same markings and weight scale as normal oil soget what you have been running if you can find it. 15 w 40 is most common which I believe is to thick for a low pressure engine.
I run Mobil 1 in my 250, the cam is stock along with stock valve springs so I don't worry about the cam. If you have race valve springs with high seat pressures, then you should consider extra protection. Race springs have way more seat pressure then yours do at full valve lift, and its the heavy loads placed on cams by spring pressure that need all the extra protection.
New car manufacturers had the zinc removed from oils due to converter plugging and eroding. They could not meet emission standards with bad converters. All new cars use some sort of roller lifter which do not require zinc, so it made since to remove it. The percentage of older cars with flat tappet cams is so small, the oil guys went along with it.
Joe
