Do you have a tach hooked up in your truck? If so, what is your fast idle rpm and curb idle rpm?(curb idle would be as if you were driving for long enough that your choke is fully open and you are idling at a stoplight.)
What was your vacuum gauge reading when you checked? Or do you still need to find a suitable vacuum source?
The info you provided about the engine sputtering wanting to die until you let off the gas pedal and it will idle seems to me like you are getting to much fuel and running rich. Can you pull the plugs and post pictures of them? Mainly concerned with the end that is inside the engine. Dont clean em or wipe em off, just pull them one at a time and snap a picture with good light so that you can see it clearly. If you were flooding the engine real bad for more than a short while, the plugs will appear dark and may even be wet with fuel. You could possibly end up with fuel in your oil if they are wet with fuel. A simple way to determine this is to pull the dipstick after it has sat for a while, like overnight, and smell the oil on the end. If you smell gas fumes or anything other than just oil, there's a cause for concern. If not, then you arent flooding the engine which is good.
Another item to check is your vacuum canister that is attached to your distributor. You want to be sure that the diaphragm inside of the canister isnt ruptured. This would cause a vacuum leak in addition to not functioning properly to advance your timing when it should be.
This happened to me. Took me a while to figure it out. And it happened by chance. I had my distributor apart because I was hunting down something weird going on with my engine. So I was systematically going through all of the possible things that could be causing my issue. Anyways, when I had the cap and rotor off I thought I might as well clean up the top of the distributor where the module and other parts are. When I pulled out the vacuum canister I wanted to know how the thing worked. So I tried moving the arm that is attached to the diaphragm inside of it. I realized it had some serious tension on it so I grabbed a screwdriver and used the shank to push in the arm, well about an ounce of the most varnished, rusted, and foul smelling gas shot out of the little nipple on the can across my work bench. I remember thinking to myself, "why is gas coming out of my distributor?" LOL. Well it was a result of multiple compounding issues I had at the same time. (That was a tough year for me. But I learned alot about my truck.
) also, the gas inside destroyed the diaphragm and ruptured it, creating a vacuum leak and not advancing my timing properly. Another reason it took so long to find is because people always say" Disconnect the distributor vacuum advance when adjusting timing." Which is 100% true. But, if you want to know your true timing at any one point along the rpm range, leave it connected. Then you are seeing how the engine is advancing based on rpms and vacuum. So, I say CHECK your timing with and without the distributor advance canister connected. But always ADJUST your timing with it disconnected and the source capped off.