Mmmmm.... I stuck my neck out and made an educated guess at the .007" number.. so, well OK, maybe .006" for 302 chambers. .0053 just computes out too small, even for the early 273 head chambers, which is what they are talking about with that .0053" number. If you milled off .030", then the difference is only 1-1.5 cc's anyway. So, let's compromise on .006" milling per cc and you'll be darned close. If you over do it with milling, it is only going to make it better. I can't imagine much price difference if any to mill .020" vs .030".
IMHO, I don't think I;'d bother with those eBay rods and pistons. There are better ways to spend those $$.