DON'T!!!
AGAIN, DON'T!!! CUT THE FRAME!!!
Change the bumper if you have to, but never cut the frame...
I'll cut up the frame on a full on wheeler, but doing that to anything else is a bad idea... If you decide to change it later, you have to fix your frame... Once you cut the frame, nothing stock will just bolt back up...
Sorry about being such a drama queen about this, but I have gone in after someone else and had to deal with this... If the bumper gets hit and needs replaced, you now have to build everything to fit the cut frame...
If it doesn't bolt up right, it's the wrong part... Or the company you bought it from are hacks... If you got it used, change the part to fit the truck...