Seat belts too short?

Finally got my seat back into the 79 and the seatbelts are not not long enough. The new upholstery didn't have the same passthroughs as the original so I figured go around. Unfortunately they're now far too short with the seat all the way back (where it needs to be for comfortable driving). Any recommendations? The new cover is leather so it's not ideal to have to cut into it in case of mistakes.

Cover is from Freewheelin Bronco with new foam