I have 180k on my 2002 F-250 and I got the phoenix for it, it makes a lot more power now
. I think your 99 will be just fine. Bill and Cody both have trucks with over 200k and they are having a "buildoff" with them, seeing how much power they can make. they might even be close to 250k miles, I would have to check the thread again. so don't worry about it, you wont get the hp that a late 99 will get, but you will still have a noticeable improvement.