If you haven't already purchased the large tree, then I would suggest you reconsider the size you're planting. The work involved in planting a tree that size, plus the possible rootbound state (if it's in a black pot) or losing lots of roots (dug up to ball and burlap) means you have a harder time straightening the wrapped woody roots and planting at the correct level.
I would plant no more than a 5-gallon tree, and this is why: You can soak that 5 gallon tree in a bucket of Garrett juice (compost tea) and water and disentangle the wrapped roots. They'll be more supple at that size. Visit the Library of Organic Advice - there are lots of tips on tree planting. Here is a newsletter overview:
https://www.dirtdoctor.com/garden/Tree-Newsletter-Planting-Series_vq1712.htmPlant the tree in native soil, keep it high in the shallow hole, stake out the roots with twigs or something that can biodegrade eventually, and don't build a mound over it or stake it. Keep it watered appropriately for the first couple of years, and you'll find that the five-gallon tree planted properly will soon reach the size (within a couple of years) of the much more expensive and difficult-to-plant large tree. The large tree won't put on size for quite a while as it adjusts to being transplanted while the smaller plant will take off much more quickly when properly planted.
It may seem counter-intuitive, but given the choice of a 1-gallon, 5-gallon, 10-gallon or 50+ gallon tree to plant, I will go with one of the smaller ones because the plant itself will be easier to work with and adapt to the new spot more quickly without the trauma of trying to plant the large tree.
Whatever the pot size, brush off the soil on top and expose the portion of the trunk where it swells - the root flare should be above ground, and even planting on a slope, you will probably want to create enough of a ledge so that it starts out with the flare exposed on all sides. Growing on a side hill may mean that in the future you'll have to return and uncover the back side of the tree's flare, but the tree may be fine with the downhill side exposed adequately.
This article discusses how trees end up too deep in the pot to begin with.This would be a good question to ask on the Sunday radio program, if you're inclined to call in. Chances are this question arises for others and it doesn't get asked often. I hope you'll call in.