A few thoughts,

I wouldn't stress over the appearance of the oil in such a short time. Motor oils have heat activated additives that change appearance with normal use. Nothing to be concerned about. Here's a video from GM which I find helpful in showing folks it's very difficult to judge motor oil useful life based on color alone. I'll start the video at 1:05min to avoid the marketing on GM's OLM system. -> https://youtu.be/oi_J4SZUlSo?t=1m5s

I also agree with others that 20W-50 may be overkill. Don't know the specifics of your build, but as I recall, the general rule is 9-to-10psi per 1,000rpms. You may be able to accomplish that with a decent 10W-30 and free up a pony or two in the process.

Also, plenty of full synthetics with lots of Zinc in them. I'd also agree that in a seasonal driven muscle car application, synthetic may not provide a meaningful advantage. There's lots of good options commonly available. With an old school cam grind, I'd consider a 10W-30 Rotella or even a Quaker State DEFY 10W-30. With a newer fast-rate flat tappet I'd go with the specialty muscle/race oil.

Also, there is one area I can think of where a top tier PAO synthetic may be advantageous in a muscle car application. That is PAO synthetics have shown an increased ability to stay within their viscosity grade when fuel dilution is a potential occurrence like for example in carbureted engines. But you won't really be able to tell you'r degree of fuel dilution without used oil analysis testing.

Good Luck