There will be a small gap between the headlight and top of the bumper, as you can see. However the gap is not a huge one, it's about the same as what it would be if you had the 01-03 header panel with accessories.
However, considering that to change bulbs in the turn signal housing you need to remove two bolts within said gap, it's a non issue. I have seen people completely eliminate this gap, however. Thankfully, the 'bumper line' on the 98-00 rangers are very similar to the 01-03, which makes this possible. Very popular swap, too.
Installing one of these bumpers is as easy as removing the old one and popping the new bumper in it's place. No mods to the truck itself are required. Have a buddy to help you; these bumpers are awkward to work with. The nut size should be 19mm or 3/4. Penetrating oil and a breaker bar are your friends. Maybe an extension. If you've got an impact, use it. The bolts may be rusty, depending where you live and all.
Btw, it's worth snagging the powerdome hood (if it has one) assuming it's in good shape. Another bolt on swap that looks awesome, which is also popular.
The truck on the right doesn't have a powerdome hood, the one on the left does. And yeah I know, I'm whoring my pics here, lol. If nothing else, it'll give you an idea of what you can expect from swapping out those parts.