I would just give them a good visual going over and maybe check them with a tape to measure, but i bet they would fit right in.

Look at the mounting points in your car for the bottom seat and the seat back. If they are the same then they ought to fit in.

Very few of the US cars had the adjustable rear seat (well, OK, maybe a lot of the 560 SEL did) but something that was somewhat common on the 420 was the rear sunscreen that would raise and lower. I am not sure if the seat you are looking at has them but if so you might need to get it to prevent a gap at the top of your seat.

I once crammed a rear seat from a 1971 280 SEL into a 1975 230E. It was tight, but it went in. Since these cars you are dealing with are both 126 cars it should fit fine.

Another way to check this out is to see if the yard has a 1983 300 SD in it. Then you could take the seats out of the 420 and try them in the yards' 300!

Pooka