In your answer to Ken Smith in St Pete.  you mentioned black smoke was oil smoke. I was a diesel and gas engine mechanic for about 30 years, 15 of witch was with International Harvester motor truck division.Was brought up in the 60's- 70's, 350's, 409's, 454's, 396's, 440's, muscle cars and trucks running rich!Black smoke has always been over fueling weather diesel or gasoline engines. Blue smoke was oil and  lean diesels.White smoke was either water/antifreeze ( you can taste that smell of glycol) or in the case of IHC's DV-550  Diesels,it is a lean and cold diesel starting in the winter with out the swirl destroyer working.If the fumes burn your eyes it is in most all cases fuel. If it just smells like hot oil on a manifold it is oil.I am guessing that this 350 is electronicaly injected, and not carburated.( If it is carburated I hope it is not a rot chester!) It sounds like someone is telling the computer its cold.  A guess for all the thermostat changes.I know this is an expensive test but, have they changed all the injectors? Not just one or two, it sounds like they are hanging up if they are not being told by the computer to run rich.  I have had diesels after running flat out for 2 hours on the road come to an exit and have one or two hang up and run ragged for the rest of a trip. In those cases the injecters got scored internaly, Not the drivers falt!  Weather it is fuel or poor tolernces internaly causing them to hang up.You could put the injectors on a rack and test them to make sure. I would call Pinallas Diesel Service and ask his advice. I do not know them and they have two reviews 1 bad, and 1 good.Now that I think of it that 350 is a GM right? call Dick Norris in Clearwater up on US 19N. They are good people, and I am sure the service manager will be able to give you some good advice on the injection system. Also he may know of any problems that have come about with the new gas. They may also do inhouse injection work and be able to test your injectorsI hope this helps with your problem.  Papa Tait, Seekonk,Ma.