I think I just walked into a bar fight.
Anyways (ducks beer bottle thown by Speeds8erM-1) the way to remove the seats is easy, just unbolt anything that holds the seats in. The seat's actuall weight is minor, but after you take out the brackets, seatbelt assemblies (ducks chair thrown by kdezines) the weight starts to add up. While you are at it, might want to take the backing off the carpet and remove some the the sound deadening material. You'll drop over 100 lbs easy with all this.
As far as the finish (dodges kdezines as he is thrown by Darkwriter77 and Phatdaddy) you might want to keep it light if you want weight reduction. You could just grind down all studs and have carpet molded to the surface of the floorboard.