Chevron Delo ELC. You don't want anything containing silicates if keeping and maintaining your system to cool properly over a long span of time.
Napa has it. https://www.napaonline.com/en/p/CVZDELOCOOLANT
Very nice! Doesn't necessarily guarantee it will adequately cool a Big Block Chevy! I've had to add Auxiliary Radiators to several BB equipped vehicles in the past.......348/409 "W" engines are even worse!