From my experience, I would suggest getting a set of matching bucket seats, the same as your fronts. Doesn't matter what color or material they are. Then, get your upholstery guy to modify the cover off of the rear seat that you have now, to fit the buckets. This way, you don't have to start modifying & welding the original rear seat frame, & your rear seat cover is about 3/4's the way towards fitting the buckets, already. It's virtually the same pattern. You will have the same material that matches the front seats. The upholstery guy will be just using the 'faces' of the rear seat, to mod to fit the new rear buckets. Factory leather seats only have leather on the faces. The part you sit on. Everything else is vinyl. That would be the sides & backs. This makes the mods fairly easy, as the vinyl is readily available, as opposed to finding the original leather (or fabric, whatever is the case)
This can be up there on the 'difficulty of mod' scale, but very do-able, if your upholstery guy is fairly profficient. Just time consuming. Measure & test fit a bunch, cut once!
If you get some buckets that are pretty beat up, you can even mod the bench foam to repair what is needed to be done to the buckets. That's already 90% of the shape of the buckets, anyhow.
If someone came to my shop, this is the way I would suggest doing this mod. Slicing & dicing your existing rear seat to match the front, to me anyhow, seems to be a lot more work with a result that may not look as proffesional, or as factory as a set of modded front buckets would.
To mount the buckets in the rear, you would have to get a second set of rear seat uprights. (You already have one!) This would be because the rear floor is not the same as the front, & I don't think that the uprights from the front would fit the rear floor contours. The tracks on the new rear buckets would bolt to the rear seat uprights, retaining the ability for them to move back & forth. If the new buckets are electric, the wiring for those is pretty easy to make work.
As with any bench-to-bucket conversion, you'd have to drill 4 holes in the floor for the inside rear seat uprights.
Seems like a lot of work, but I think that finished results would look a lot better than trying to turn a bench into buckets.
