I have a pair of 452:s in the basement, fluxed, cleaned and ready to get new guides, valve job and porting. I've been offered to buy them for $350 but after I put the machine work into them, they're another $6-700.
just me, but why not
By the time you get a 906 or a 452 set of iron heads ready to make 500 horsepower, with a mild cam (or a radical cam for that matter), you could have bought set of better aluminum heads. Well, unless you own a head porting shop.