Checked fuel pressure? Pull gas cap off of the filler neck to vent the tanks? Sometimes those anti-siphon valves can stick when the gas sitting in them dries out and gets gummy. More than one tank vent has gotten clogged from sitting with spider nests and what nots.

also Ck for cracked spark plugs from being pulled. If new should be good to go. If not hit on installation also you didn’t partially hit you’re fuel shut offs. If you have them in this whole process. Seems like fuel delivery or ignition prob to me. Good luck

Ground straps and all connections and including main Mercury plug all good. As mentioned possible cracked plugs. Fire it up in the dark and check for any arcing etc. just a thought. I've identified some electrical issues in the dark. I have a new DUI with plug wires if your interested in purchasing or just to try. Just pay shipping. Process of elimination. New plugs and believe it or not it wouldn't be the first time even fuel at the pump can be contaminated with water. Happened up north here and cost the station a brand new Diesel engine.