When I put the floor pans in my '65 Coronet, the hardest part was getting the underside of the car completely clean before doing anything else. I did both fronts by laying the new pans on the old ones and then after marking, carefully cutting out the old ones so I could butt weld the new ones in. Flanging and then welding would be much easier, but I think that it would trap water. You will have to drill out the spot welds where the floor sits on the front frame section, but it's not hard. You should clean and rustproof the inside of the frame rails at that time.