What are your rod and main bearing clearances? If less than .003” xW40 will do ya. Did you install a high volume pump with the high pressure spring and pinned pump drive?
The issue I have with 20w50 is the time it takes to reach the rockers then run down onto the camshaft.
A scene often witnessed on the beach are big blocks sending their crankshafts to the bottom of the lake because the owners started the engines and went to full throttle 30 seconds later to show off to the girls.
The cautionary tale is give your BBC some warm up time at moderate load to warm up the oil.
For continuous operation above 3,500 rpm, get rid of the stock harmonic balancer.
A stock 20 year old one won’t work properly and is a cause of cranks breaking right behind number two rod journal.
Nowadays, I wouldn’t be adverse to using 5w50 Dexos R or one with an A3/B4 approval. Their HTHS is high enough for your bearing clearances and the VIIs will keep the oil pressure high when it gets hot.
Another engine oil proven over time is good old $4 Delo 400 SAE 40 as long as start up temperatures are above 40F.