I would imagine with old padding and springs you are still going to have seats that sink down more than they should. You need to get the shell off the seatback somehow and strip down both seats. Then I would probably repair the broken spring in the seatback instead of replacing the whole thing, if possible. There are ways to clamp the broken ends together with metal tubing. Then pick the best pads to put back on. Give it a test sit. If it sags a lot you either need new springs, or do the pool noodle trick. You can recover the seat on your own but you might get lumps. A pro will probably do a better job, unless you have done it before.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma