honestly with the money youll have in valvesprings, retainers, valves and labour....
you wouldnt go ahead and bolt old worn heads on a motor you intend to use yourself would you...?
and wrench turner is right in saying that there is a lot of edges.. 400 have bigger bore than a 305 so you need some pocket porting also.. now it would be much cheaper, and give you way way better results to buy a set of Dart or Motown heads..
and if you cant afford the propper heads now then save up your money till you can.. remember that the heads are where the horsepower is made
hope it helps