My name is Jace and my 1968 Ford F-250 with a.

Hi my name is Jace and my 1968 ford f250 with a 390 Fe just drove about 900 miles pulling a U haul trailer. Keeping it at about 2500 rpm it never overheated or had any problems. We get to our destination and it starts really running rough the idle is going up and down and it blows black smoke and is just running terrible. Our first thought was bad gas because it was right after I switched the tank. So my brother messed with the carb a little bit and it kinda helped run a little better but I decided to just burn the gas off and see what that does. Now all the gas is gone and it is running okay still up and down with the idle when it gets warmed up. Now I have it at a mechanics shop and they did points cap and rotor and coil. It is still doing the up and down idle but only blows black smoke when I rev it up. Its not terrible black smoke. But its never done that before. The mechanics are telling me a vacuum leak in the intake or the distributor is lose and getting worn out. What other possibilities might it be sir. Thank you for your time and look forward to your advice.