It would've helped if you measured both bolts and posted the lengths. I suspect that you have recessed mount brakes which take nuts like
these front and back.
If so, you may try this trick. Try the front brake (longer bolt) in the back, and see if the rear fits the front deep enough to enter the inside of the steerer by more than 6-8mm. If so, you can mount them that way using a super long nut or my coupler trick for the fork.
The other alternative is to look for brakes with standard (long)bolts. These are usually available in long reach versions, so it shouldn't bee too hard. But even then you may find that the front bolt is still too short, and need to drill the fork or use a coupler.