The 993 casting went out of production in the 70`s. However, Goodwrench later picked the casting back up on there targetmaster crate engines, you should see "hencho en mexico" on the casting, meaning, made in mexico.
These heads are heavy casted, 76cc chamber, 1.94 intake 1.50 exhaust.
305`s never came with these heads from the factory, 305`s have 1.71 valves and 1.84 inch valves on the intake side, since a larger valve will likely hit the small bore. The 400 small block was made from 1970 to 1980, so what you have is heads off a 350 if your certain they were made after 1985. Look on the block, if you see the same mexico casting, you have a target master block as well.