the 587, 675 closed chambers, 915's are open i believe ? The 675's are 76 cc's. I believe not sure? my heads were milled about .065" to get around 60cc. What is the max millage?

The 051's already have a small chamber 58cc or something. Can't take off too much on a 051.

I used to have them cut until they were nearly closed chamber .09 or so. you have to watch the intake valve seat as it protrudes into the chamber somewhat. I ran felpro permatorque head gaskets and they seemed fine. but this was a race car not a street car so if I had had problems I wouldn't of been stranded. It night of made me a little braver.

The only factory closed chamber heads
315- and another 273 head I don't remember the number now came on 273s 64-66 they are typically about 60cc
920 was one year only head for both 273 and 318s in 67 then they went open chamber.920s are also about 60cc
302-714 85-91 318s used these, they are the high swirl head and run about 62cc
And of course the magnum heads witch are all basically the same no matter the number and they are typically around 62cc

All other factory heads are open chamber cept mabey some oddball motor home head or industrial or something obscure like that.

Open chamber 340-360 heads are normally around 72cc and
273-318 open chamber heads are around 67cc.

Every ones deffnition of "too much off" is going to be different. Some people are OK if it lasts a season of drag raceing before they crack, some people want them to last forever, some people it is too much off if you have to mill the intake surface or if you get the intake side so thin it is hard to seal.