Sometimes an engine bay/motor/manifold/carb gets got. Hotter than it used to get for some reason, and evaporates/boils out the fuel. This is usually a problem after a motor is shut off and "heat soaks" thru the block/manifold and into carb body. That's why the phenolic spacers are used. I don't know if that is your problem unless you have noticed a huge rise in temp gauge. The V6 is known for extremely good cooling.
Heat can also cause some electrical components to fade. BE SURE your points are set properly.
I don't know the history of this. Did it run good under load until recently, then started to die, then you did that work? I'm thinking it was carb or pump and one or both are still the problem. You need to carefully recheck float level and float drop to specs. Be accurate. Make sure float is not hanging up on side of bowl. Double check all the components, I'E. balls, accel pump skirt, needle and seat, orientation of gaskets and if you cleaned it good enough. What carb do you have? WW ? Rochester? Holley? 2bbl?
Try to remember a moment in the process when something changed. That may give you a clue. Keep testing to be sure it's getting adequate fuel. Go with the mechanical fuel pump if you are sure it's OK. Those pumps are hard to find, right? Make sure the flow test is long enough to detect a blockage forming over time. Remember, if a fuel filters are clogged it will cause what you have. Non fuel quality hoses will collapse.
Take off the gas cap in the near future and run up hill, etc., with it off. If tank/cap is not venting, it will pull a vacuum on the whole delivery system and after a short while...........no gas to carb! But it should do that during idle/rev testing? Gas blowing out somewhere or air puff when cap is removed is a tell tale.
Also because it dies under load, check ALL vacuum ports, hoses and hole plugs for leaks.
Can't have any leaks.
Keep reporting any findings so we can all keep thinking on it. Just don't keep any secrets
