The '66 should have 516s, but may have small valve 915s especially if it's a later production assembly. I've personally seen both on original engines. Both are closed chamber. Both respond very well to replacing the exhaust valves with 1.74s. Early engine has the forged crank and more swap friendly pulleys and brackets - again - assuming no AC.
The later will have 452s or a variant of them - open chamber. Lower compression but if either engine can be heard run in the car(s), and compression tests done, they are worth his price all day long.
