The local rules here in Vancouver (Canada) are that the vehicle is tested to the chassis year requirements.
Since the 4.3 produced 1/10 the pollutants of the 2.6, it was an easy pass.
If tested to the engine year, it would have passed too.
An engine/exhaust has to be pretty suspect here to fail.

I took mine through the test (dyno'd, 5 gasses) right after it was running, just to see if I was due for a "surprise" next year, and it did just fine. The tech's updated the records to show the new engine size, and were curious about the swap, but otherwise nothing caused any problems.