Feets, I only get that smell when my truck is cold. A few minutes and the rotten egg smell goes away. Thinking you are experiencing the same thing when a Customers' car gets fired up out in the lot and gets rolled into your shop.

If I had to pin point when the smell goes away, I'd say it's when the O2 sensor has enough exhaust heat in it to go open loop.

Also trying to say: throw those convertors in the garbage, live with the smell, and enjoy your 20hp and +2mpg you'll be getting.