IIRC, you can get a rear 02 sensor sim and install it on the 99's and not set a light. I have one in my 00 GT and no issues. Since the upstream 02 does all of the work it shouldn't be affected since it is before the cat. The rear one just reports on the condition/efficiency of the cat from what I recall.

Backpressure isn't really any concern on these modern cars.