215/70r14 is 26.9" tall and 8.7" wide. 275/60r15 is 28" tall and 11" wide . So your 1/2" taller from center. No big deal. You are also 1.15" wider per side, but call it 1 1/8" for simple measurements and fudge factor. Probably no big deal here either, but lets check.
With a 4" backspace rim you would have the same zero offset on a 15x8 as your 14x7 with 3.5" backspace and relating the above measurements would be simple tape exercise of adding in the extra dimensions against your current combo.
However, your chosen rim has 4.44" backspace which means it is .44 offset from zero. So you have to add .44 to the 1.15 for 1.59" That means you new combo will put the tire sidewall 1 1/2" closer to the leaf spring than your current combo. On the outside, we take out 1.15 wider tire and subtract .44 and get .71. That means your new combo will be just shy of 3/4" wider towards the wheel opening side than your current combo. So you can check: if you currently have more than 1.5" from the sidewall to the rear segment of the spring pack, (don't forget E body springs are splayed wider in back), then your new combo will fit. Some on the outside, if you have more than 3/4" to the fender lip, it fits.
Now that you know how these dimensions compare, the change to a 4.5" backspace rim would add an extra .06 or 1/16" measurement to the spring side and subtract the same 1/16" from the fender side and you can see the difference between the 4.5 rim and the 4.44 rim.
Or you find a friend with the same combo, borrow his rim, and test fit it, have a beer, shoot the breeze.
