how big was the peach tree Mahonri? Dwarf, standard?? I'm assuming dwarf since you said there was a graft section.
rule of thumb is to plant them as deep as the root ball / container they are in. We planted 7 dwarf trees last spring (1 peach, 3 pear, 1 cherry, 2 apple) and I got them off of millers - 6 arrived 5 feet tall and the one apple arrived about 3.5 feet tall. All were bare root stock, but had branched on all but the pears. We planted as directed - about 2 feet around per hole but only as deep to keep the graft above the ground but enough to spread the roots out (probably about 1.5 feet deep).
Ours said to ensure they were "mounded" and not flat so that water would run off of them and not pool, so we actually had to build it up a bit. We used tree and shrub soil and mixed it in with our regular soil as well to acclimate the trees.
Our trees now are about a foot or 2 higher than when we first planted them
and should produce this year for us (they were 1 to 1.5 years old when we got them and produce their 2nd year).
My peach (elberta) is just now starting to show some minor blooms on it, moreso the leaves. So I wouldn't worry too much yet - if you can, try to break off a piece in an area that wont hurt it and see if the tree's still alive. If it snaps easily, its dead - if it bends, its still thriving!