I also have an older swenson Vbox salter. There was never a bottom plate on these boxes. As far as repairing I just had mine rewelded on the bottom with new metal coming 4 inches down the side and including a new lip that the chain rides on underneath. They unfortunately are not bolted on this unit. You can glue strips of marine grade plywood on top of that lip to help wear from the chain and also it keeps salt from laying on the lip underneath that will eventially rot it off if not protected. The plywood is a lot cheaper to replace than replacing the lip. Also make sure if you fork the salter out of the truck or store it on pallots etc. keep it off the cement and the ground as this will also help eliminate the bottom from rusting. If it gets weak it will collapse the lip and the chain will bind or break Just my two cents!!
What I did with mine was take a cutting torch and cut it at the open end instead of trying to get the pins out. It will then just pull out. A lot quicker and less frustrating. You may have to cut it at the front also so you can pull it from the top and the bottom as it rotates around the front spindle gears and gets hung up.
Hope this helps
Usually you have to cut the older chains. If the unit is running the motor will turn the chain right out. If not it will be a bit more work. I have had to take them out link by link after they broke and jammed in the tray. Not a fun job.