It seems the spokes are too long. Two possibilities I can think of:
a) If the rim had been built into a front wheel before, and you are using the same spokes, it's possible that the rear and front hubs have dimensions different enough that the recommended 2 mm margin of error on spoke length is insufficient. The main variable affecting spoke length (besides the ERD, but here I'm assuming the rim stays constant) will be the flange radius (1 mm less means about 1 mm less on radial builds with a smaller difference the more crossings you have), but a big difference in the flange offset from the center of the hub can have a significant impact as well. Use a spoke calculator.
b) you're doing a different number of crossings. This will affect the the required spoke length bigly.