Of course they still work-I don't think that was ever a question in this discussion. The fact is that most hot rodders operate within budgetary constraints, and want to get the most bang for the buck. If they're not able to plunk down $1500 for a new set of heads at once, then yes, a freshened set of factory heads (even later model smog heads) will work satisfactorily...even better if they've been ported on by someone with some patience and a flow bench.
All that being said, if the car isn't set up well, no amount of grunt will be useful for anything other than making expensive clouds (which does have it's own intrinsic entertainment value) of smoke or breaking downwind parts (...not so much on the intrinsic value part...)