OK,
Sounds like you have spark...
Do you have fuel? Above I gave you a breakdown, and instructed you to poor some fresh gas down the throat of the carburetor. Did you try pooring a small amount of gas down the carb?
If you have spark and fuel, and your engine still won't start... A compression test is in order. Several people above have hinted at reasons why there might not be compression, including stuck valves.
The gmc 6 cylinders are solid motors, and are known to run even with 3 dead cylinders.
I assume when you changed all the ignition components you changed them properly. Make sure you have a bright spark at the points, and a bright spark at the plugs. If not readjust your points. There is a possibility you recieved a bad cap or rotor. It is also possible you have a bad ignition coil.
Are you trying to start your truck with 6 volts or 12?
Are you aware that a 53 gmc would originally be 6 volt positive ground?
Do you have a 6 or 12 volt coil?
If 12 volts, Do you have an ignition resistor?
Please answer these questions if you can.
John
