Nah, the engines are all the same dimensions regardless of displacement and block material, Gen III and IV. Same with the drivetrain, barring the 6L80 vs. 4L60 differences. What does make a difference (and is the real PITA) is 4WD/AWD versus RWD. Obviously, there's much less wiggle room with a 4WD/AWD, but there's still plenty of room for headers. Yes, 4WD and AWD can be considered the same externally since the difference is the internals. The issue isn't that you have a 6.2, it's that you have AWD and some headers weren't designed to clear the front prop shaft, which is mostly a Y-pipe issue. Each header manufacturer has their own solution. Some designed their headers to fit 2WD and 4WD/AWD and may just require a drivetrain-specific Y-pipe or require modding of the standard 2WD Y-pipe when used on a 4WD/AWD. Others may have completely different headers made up to accommodate each configuration.
More specific to your application, the 6.2 only came with the 6L80 trans, which is larger than the 4L60. The 6.2 also has a larger exhaust. So, it's pretty much a double-whammy for an aftermarket header company to contend with if they were trying to make one set of headers fit all powertrain configurations.