This is a $11 Dorman "coolant recovery" tank (read it is like a modern closed system vs "overflow")
I used a spacer to match the offset from core support on one of the 2 attachment screws. The hole for the hose from radiator is already in the core support. Coolant recovery tanks require a "closed system" cap. You fill radiator completely up vs leaving them 3" low for coolant expansion. As coolant heats up it expands into the "recovery" tank and when the motor cools, it is drawn back into radiator. The hose from radiator goes into the bottom of the tank vs the top of a "overflow" only tank so it can be drawn back into radiator, like a modern cooling system.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
The tank noted that would fit inside PS fender is for a 75 Nova ($45 on Summit). I have one in my 67 Camaro. My 72 C10 has the vapor canister in that area so I elected to go Dorman route on the truck