The first generation of the Seville was right in the middle of the smog-era— 1975-1979. This car was based on the K-body architecture which itself was based on the X-body (Nova, Omega, etc.) but was on a longer wheelbase and built for a softer ride.
For 1980, the Seville was redesigned to front wheel drive orientation. This time it was loosely based on the E-body (Toronado, Eldorado, etc.), but was still known as the K-body. Having a slant back shape, This car was quite a radical departure from the previous boxy Seville.
A new 368 V8 based on the classic Cadillac big block was now standard in 1980, though it was very underpowered at 145hp. Almost more infamous than the GM diesels was the new 6.0 V8-6-4 in 1981, which were promptly converted to standard V8s at the request of frustrated owners. The Olds 350 diesel was still available through 1985. A 4.1L Buick V6 was available for ’81/’82. The new standard for ’82 onward was the HT4100 aluminum block V8. The end of this was generation was 1985.