First, a 275/60/15 isn’t supposed to be mounted on a 7” wide rim. Cue a whole bunch of guys saying they’ve done dumber stuff than that and haven’t died yet. But whatever, that’s what BFG says so take it for what it’s worth, they’d probably rather sell you tires than not and that’s still their recommendation.
As for the 15x7” with the 4.25” backspace- that’s the same offset (+6) as a 15x8 with a 4.75” backspace, which definitely won’t work. Those wheels have the same centerline (same offset). So sure, the backspace is less but the tire will hang over the rim more. Will it be squeezed because of the narrower rim? Yes. Will it be enough to work? Haven’t tried it. My suspicion is no, but I don’t know exactly how much those tires will be squeezed.
