If you are going to replace floor pans, atleast do it right. If you think that your but welds are that strong, you better think again. You have to have the panels fit perfectly to get a strong enough weld, specially for the floor. The factory lapped the joints and sealed most with seam sealer. Since most people don't have access to a spot welder to do them exactly like the factory. The best bet is the use of plug welds, done with a mig welder. Apply weld thru primer first, and seam sealer after you are done.
I would not have a car that someone had installed floor pans by but welding them. I would be cutting them out and replacing them immediatly.