What they said.
And, why not check that timing, with the engine off. Do a search for static timing, or just static. There are many posts on this.
Also, is the carb mixture set right?
Does the accel pump work?
If all the above is copacetic, a great trick for finding vacuum leaks that I learned right here at stove bolt, is to use a propane torch. Not lit, and pass it over the intake manifold joints, carb base, etc. Its gotta be a big leak, if the above has been checked, and you are still having the problem. Don't do it inside, and don't get on fire.
A vacuum gauge would be a good diagnostic tool. Cheap, and really works well. Once you are running right, you can re check your timing while the truck is at idle too. I never knew just how useful they were, until I got me a stovebolt.
