First time engine build won't fire

The gas spitting back out of the carb indicates that the ignition timing is way out of time, OR you have the rotor in the distributor indexed wrong. And if the cam timing is off, then setting ignition timing to the damper mark will be wrong. As said, if the timing is right and there is a bit of fuel in the engine, it will fire.

- Check spark as in port 8. Checking for the spark to jump a spark plug gap in open air is not a valid test at all. The spark has to jump at least 1/4" in open air to be able to jump a spark plug gap in a compressed fuel-air mixture.
- Pull the distributor cap with the damper at 0 TDC; the rotor should be pointing directly at, or slightly CCW from, the #1 or #6 spark tower.
- Pull the valve cover as described in post 18 and look at the valves. When the timing mark on the damper passes 0 on the timing cover, on either #1 or #6, the exhaust valve should be closing and the intake opening. On the next rotation of the damper mark past the 0 mark, the other of these 2 cylinder's valves should do this. If this exhaust close/ intake open does ot happen right around the 0 mark, then the cam timing is off.