My brother has a 220D -72 w115 with w123 front seats. The mounting on the floor is little bit different and seats eat some leg room of back seat passenger.
Welding work may be required to do this properly.
123 seats are better shaped but they are not as durable as 115 seats, I think. There is coil springs under the 115 seats, it will take decades them to break. Maybe the later design was less expensive. I havenīt found real springs under the 123 seats, only some kind of leafs...and the left side of drivers seat will go down permanently in short time for M-B and shape is lost. Just my opinion