A set of tricked out stock heads, ported and reworked to optimum will still be down forty horsepower compared to a comparable port size aftermarket head right out of the box. There is that much difference in technology advanced in the last thirty years or so.
That being said, there is a sizable following of persons with money to buy what they want that states if it isn't bone stock it has little to no value other than as a restoration project to bring it back to stock. Get one the way the factory made it with the right paper tags on the springs and chalk marks on the fire wall and they will pay through the nose for a car that cost less than $3400 when new.
As another note of caution. Stock Chevy parts and V-8 cores are rapidly aging out of junk yards (most have nothing older than ten years in their yard). They treat old Chevy 350's as though they are nothing but scrap metal. Crushing and shipping everything to Korea to make Kias and Hundias or to India to make "smart" cars. If you do find a set of '186 heads I would buy them, rebuild them, then I would store them in a plastic bag awaiting your final sale of the car. This is because the older heads will increase the value while aluminum heads left unpainted will decrease the value.