Mercury Sable won' start?

I am working on a 1990 Mercury Sable. with 3.8 liter V6, for my elderly mother in law. The car ran fine. It leaked coolant, so I suspected a head gasket. Years ago I fixed my ’90 tbird with the same engine same problem. After reassembly, the car started, but ran very poorly, and very rich, with black smoke. The engine would not rev over 4000 rpm. I thought the injectors were not working. After testing resistance and voltage I found they were. I changed the fuel pressure regulator, then the car would not start at all. It tries to start, almost, like the timing is off. I did not move the distributor during my gasket replacement. I live about 200 miles away and work on it once a month or so. I want to get it running as it was before the change, I am lost at what may be wrong. Any ideas would be welcome. Thank you in advance. earlt57@gmail.com