If you unbolt the bumper and then try to fit it into the proper position (which slightly overhangs the lower lip of the grille -> BTW) it will be obvious why the framehorns were cut. The lower side of the bumper will hit the frame and you'll never get the bumper to tuck in correctly.
Personally, I think it might be more trouble than it's worth to try to do a 100% factory restoration in a case like this..... (I don't have front frame horns on mine either, in case you haven't already noticed)
If you are new to fabrication, building some custom bumper mount brackets are a great first project... You will get to cut metal, practice welding and creating a bolt-on adapter to merge the bumper to the frame. As long as the welds are strong, it really doesn't matter if they look a little ugly. Once you're done with it you will probably paint it black anyway so they will barely be visible at all.
Depending on your desires, you can also build yourself a "hidden hitch" at this point....another good starter project.
By the way, the stuff I've learned about metalworking and fabrication all came from my build (and obviously studying what others have done). One of the reasons my build has taken so long is because almost everything that I've done also required me to buy new tools and learn new skills at the same time. If I ever decided to do another truck build (not likely!) the whole process would go a LOT faster next time around.
