The site is on a slight grade but clay retains water. Does that count as well drained? I'm a newbie where trees are concerned. I have found a great web page giving the characteristics of various rootstocks and there are only a few that are "widely adapted:' P.18, Seedling, Antonovka 313, M4 and posisbly Geneva 30. The rest either need well drained sites or need testing. I can't seem to find anyone selling heirloom, or most other cultivars for that matter, on any of these rootstocks.
The following address is the aformentioned "great web site."