The fat lip bumper design introduced in 99 has more Crush can and safety improvements over earlier years not to mention styling and internal updates as well. In my opinion 99 and 2000 are the golden years of 3rd gen design but I'm a little biased.
If you had access to a 98 donor vehicle you could probably transfer the front bumper Etc, but you would be compromising safety in the same way that you would installing an aftermarket bumper that requires cutting off the crush cans and mounting pads.
That being said, and aftermarket bumper on a 98 versus a 99 wouldn't look any different from the front, but from the back, the 98 8 1/8 liter license plate bezel gives away the age of the vehicle.
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Doesn't the ARB bumper come with it's own crumple thingies to replace the ones you remove? It's pretty well tested to work with airbags and stuff from what I've read, albeit it on a Tacoma.