Was running rich, now it's running way lean...

[arocars]

With the car running, spray water around the carb (not in it). If the idle changes, you have a vacuum leak. Starter fluid will also work.

 

[85 Cutlass Brougham]

Try flipping the fuel filter over. It is possible that you installed it wrong both times. It could also be a bad fuel pump diaphragm that somehow still pumps a little fuel. Maybe sitting too long caused a problem? Also, did you disconnect any of the myriad vacuum lines and hoses when you did the filter? If so, maybe one is now cracked and leaking a lot of vacuum. That is not hard to imagine with hoses that are 25+years old. There is a 3/8 one under the carb that runs quite a few items and is prone to cracking, as are any old vacuum tees. The plastic gets very brittle over the years. I am starting to have issues with brittle plastic on a 9 year old truck with a squeaky clean engine bay (always has been). It's possible your plastic stuff has similar issues. Have you tried disconnecting the vacuum line to the brake booster and plugging the hose to see if it runs better? When they go, it causes a big vacuum leak too.