i have built floors for older cars i have used square 3/4 steel tube and 1 by 1 tube and hook in to the door opening frame if its is not wood .then make a floor frame for seats and put in cross braces and tie this to the bottom of fire wall add steel 3/16 were the body mounts use to be make sure you are tied in good to you square steel floor frame so it is solid .on the mount i have used 1/4 flat belt for mounts. if there is no steel to hook in to try to grab as much body steel were the skin was nailed to the wood and make the frame with square tube and weld it to skin i would plug weld . 5/16 drill is a good size for the hole for the plugs. and if the door opening are wood and in good shape you can hook into you new floor by making spots to screw the old door frames to it. when frame is done i use weld thru primer and plug weld a skin to the floor frame i use 18 ga but you will have to get the body back in shape before you start and tack steel tube in to keep car square if not done doors will not open rigth. before you start make sure the door hinges are good before you start so the doors fit in the openings