I'd like to add a seatbelt to my newly acquired 580ck. Is there a generally-accepted best way to do this or should I just grab whatever seatbelt I can get my hands on and start bolting it to the metal part of the seat? Also - what's the cheapest way to replace the foam/cushion on the seat? Should I just cut some foam to fit the metal insert (not pictured) and glue faux leather down over it? Not sure where I could find said foam but I'm sure it's out there.
Unlike Ford Mustangs, there aren't a great many aftermarket companies springing up to offer restoration parts for Case backhoes. I expect you will strike out searching for offerings of replacement seats. There are numerous seats offered which require some adaptation. The other option is a reupholsterer. Last Case backhoe I had refreshed Mary McFall was still working. Perhaps I might persuade her to come out of retirement to do another. I'd bet you have a competent person you could talk into it. Sadly there is no SAE specification for any of those interior components. If the manufacturer won't supply, you're on your own. Few of us worry about correct replacements. It is more common to buy a Tractor Supply seat & adapt, or do some research & find an aftermarket seat that'll fit.
I doubt your tractor had seat belts new. I also believe you need seat belts. Again you are in a position of design build. I've done it a dozen times for various machines. If you aren't up for it, someone near can do it. Don't be surprised if people won't take the risk. Safety equipment carries a lot of liability. I build you a seat belt system, you can sue me for ALL injuries you suffer.
A junk couch cushion will have the right foam, glue it to a piece of plywood and wrap it with a vinyl and staple to the plywood, it won't look professional, or last as long as a professionally made seat, but it will probably rot in the sun before it wears out from your rear end, so not much difference in the end. Yes, any middle seat lap belt is a good place to start for a seat belt.
Seat belts have to have belting coming from the floor of the machine to the mount pain on the back of the seat and then the belt itself it attached to that. The idea is that a suspension seat could get thrown off the pedestal and you would have the seat attached to you when you stopped against something.
Each side of the seat belt on my 580se is bolted in two places: at the seat cushion bucket and on the lower seat carriage. The belts are Not bolted to the floor. They made em both ways but I’m not sure why.
My 480c has them bolted to the floor. They have been tied in a knot so long, I can't seem to get them straightened out so I can use them. I went and bought one of those suspension seats off Amazon or Ebay. You can't beat the price, you have a choice of colors, and if you are old like me, they can save your lower back. I have a lot of rocks I am digging up. Some of them you have to "worry" out of the ditch to get them out. As you are struggling with the hoe bucket to get them out, sometimes the bucket will slip off the rock. This causes the whole machine to bounce, and it is hard on your lower back. I have had a couple of episodes where it did damage my back and I had to lay off of it for awhile. I think this cushion seat with it's spring and shock absorber will help that situation.
Checkout Vevor. They have some pretty good looking seats at a reasonable price. I haven't bought one yet, but will probably get one for the 590SL.