Mulch and no plant competition. That is a great battle tool for rain problems. Leave a sign at the end of the road for woodchips wanted. Contact the local power company and town highway dept. Look for rotten hay bales.
IF you do water, water slow. A bucket with a hole drilled into it. Even 5 or 10 gallon to a mature tree is welcomed by it.
Site prep the year before planting can be real helpful too. Dig deep as you can with what you have, try to loosen up what ou put back in with whatever material you have. Even sand in clay helps, some wood chips, of course manure or peat moss is great. I rototilled, then hand dug, Did that 3 times or about 2 feet deep, then hand dug the center taproot about another 2 feet. Give it a few months to settle in some, so you don't have a underground water bucket. Post hole diggers or backhoes are alot better than my cheap labor.
When the rain comes, the surface grass and weeds compete for the moisture. Kill that off if you can Even just mowing helps.
Read on a tree site that M111 is better for lighter soils. MAking me wonder if antonovka or dolgo might be a better rootstock to get roots deeper in heavy stuff. Got 2 antonovka's at home and they seem to be making a bigger tree, not sure if that means better or not. Not bothered by flooding, hard to say about drought though. Only droptine and kerr. And the droptine is being topworked to kerr.