I have a 351w motor i had to replace the manifold we put a 302 fuel injector harness on there because i cut the old 351 harness thinking i was going carbureted but changed my mind but when i replaced the harness i got new injectors,took smog pump off and put a pulley there,i put a new idle air control valve on, new spark plugs,new fuel pump in front tank but if i can get it started i have to use starting fluid every time you have to keep your foot on gas pedal to keep it running and when it does run it shakes bad has a terrible idle and acts like its not getting enough power. There fuel going to the fuel rack, no leaks of any kind anywhere. And with the 302 harness we had to rewire a wire from the back of the harness to the front for the water temp sensor on the manifold.

 

IF,,,,,,,,,,,,,all the wiring was gone right, I would suggest you double check all the areas you worked for a vacuum hose either broken or maybe off.

If it smooths out with some action applied to the throttle pedal, very possibly a vacuum leak ,big one!

Something to keep in mind,
When adaptive memory has been lost ,the vehicle computer has to relearn a few things among which is idle speed. The best way to get that to happen is ,,,,,,,,,,,,,,,,,if you can get it to start and idle without touching the throttle pedal. It will self adjust the idle speed control and ,once that is learned you can go on to other checks.
I realize it is tough to resist touching that pedal, but if you depress it , it learns that position and setting.