Truck won’t Start

[71 C20 CS]

I did a full front end rebuild on everything except the engine and everything was fine except I forgot to put the spring back in my lower radiator hose and at some point it collapsed and all the coolant leaked out. It got pretty hot, but I don’t think anything cracked and the oil still looked good.

After that it would start but die after 5-10 seconds, but seemed to run fine for those few seconds with no smoke, now it won’t start and has a flame that shoots out the carb. The ignition and a pertronix distributor chip were replaced a couple years ago, and the coil, plugs and fuel pump are good.

Does anyone have a clue what is going on? I don’t know how getting it hot could change the timing and if the valves are toast wouldn’t I hear it or see smoke?

Any advice is greatly appreciated!