It depends on the brand of tire. They're both 70 ratio tires, while the 255's are wider than the 215's. The diameter (height) doesn't necessarily change with the change in width. As for whether the 255's will fit the wheel well, I couldn't say. I have 235's on my '48 and they're fine. You haven't said what truck you have...
255/70/15 is a size measurement. Every tire of that size will be the same height and width, regardless of brand. Same with 215/70/15, etc. There may be slight differences depending on tread design, but that's it.
