It depends on the actual size of the tire. Sometimes tires with the same treadwidth but different aspect ratios (the number you don't mention) are approved for different wheel widths.
Most 265/30-19 and 275/30-19 tires are only approved for wheel widths of 9.0-10.0 inches, so those sizes should absolutely NOT be mounted on 19x8 wheels. The problem in doing so is not only the difficulty in the mounting process, but also the fact that the too-narrow wheels creates excessive stress in the sidewalls, causing them to heat up and possibly leading to tire failure (blowouts). Don't do it! 19x9 is fine with those sizes.