Hope the video works I have to hold the gas pedal to the floor for it to start and run. Then it back fires and dies
So after it starts and is thumping (in the video), your holding the pedal to the floor the whole time?
I don't recall if they are interference engines....maybe superiorstyles knows he knows a lot about the 4.7L's. I'd figure out why the rockers were so loose, that alone could be the problem. If the rockers are too loose the valves won't open at the right time and can cause poor running or no running conditions. I had the valves adjusted too tight on a vehicle (just barely) and it wouldn't start since the valves weren't sealing (no compression). Your issue may be just the opposite where the valves are opening late (due to play from the loose rockers).