1996 Buick Regal 3.1L

I had a check engine light. Code read as 0420 (??) Auto store said cylinder misfire. I knew I needed a tuneup, so I changed plugs, wires, all three distributors, and PCV valve. Car starts and idles okay, but when I put in gear (D or R) the car sputters, chokes, and stalls. I can’t find any vacuum lines disconnected. Any ideas what the problem might be?