You can cut the headtube but you will need to ream the id of the HT to fit the bearing cup. This might make the wall too thin and crack easily. If you aren't sure if it will be a long term frame and aren't worried about it potentially cracking in the future you can buy a reinforcing ring and epoxy it on and get some matching model paint and paint the ring. If it is long term have the ring brazed and a pro repaint.