My Dad has a '70 ss396 Convertible. We built a '69 396 for it, as the origial engine was missing. We were using Valvoline VR-1 20W-50, until I found this site, and started to learn that 20W-50 may be a bit thick for that engine.
Lately, he has been using Havoline, or Pennzoil 10W-40. He drives the car about 1000 miles per year (summer months) and does an annual oil change in the spring. This is a mild hydraulic cam engine, and the car is primarily a cruiser.
Big Block Chevys are somewhat hard on cams, since they have a heavy valvetrain, high ratio rockers, and often run spring pressures that are a bit high. I would think a 15W-40 HD oil would work great in this application. So far, the passenger car 10W-40 oils are doing a good job in this 396. We have not used any GF-4 oils yet.
Out of the oils listed, Havoline 10W-40 gets my vote. No need for synthetics... just use a quality dino, and change it at the start of the "cruise season". If you want xtra protection, run a 15W-40 HD oil.