The only thing I will add to this is that with the 516 "closed chamber heads" you shouldn't have to mill much of anything to get the compression you want other than to true it up....
All the information I have been able to find on these heads on the interwebs says they have a 73.5cc chamber. All the specs I can find on the interwebs show a 1965 383 having a stock compression ration of 9.2:1.
Also, FYI, I used a pair of the same heads from a '65 383 on my 440, when I took them to the machine shop there was hairline crack at one of the exhaust valve seats. The machine shop I deal with usually works on big diesel engines, my cousin had the head sent to their guy to have the crack welded (he assured me they do it all the time on the diesels and that it wouldn't be a problem) before installing the hardened seats. The engine now has probably 3 - 4 hours of run time on it, and so far so good.
Concerning the question of exhaust valve size for your application, I really would like to hear what IQ52 would have to say about it. While I consider myself to have a fair amount of engine knowledge, this dude is like an encyclopedia.