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Internal combustion requires the the same three things that external combustion (fire) requires; Heat, Fuel, and and Oxygen.
Heat and oxygen are mostly provided by the car's compression ratio which on a 1968 motor should be 10.25:1 (which is considered high today but standard back then). With the Otto thermodynamic cycle a car uses for power we rely upon a spark for ignition (the Diesel cycle doesn't and ignites just as soon as the fuel is injected into the cylinder). Finally we use the same oxygen source that a regular fire does. The air found above the carb is one fifth by weigh pure oxygen. But a cam with wiped cam lobes will keep the valves closed so you get in neither air nor fuel. (pull both the valve covers to see if all the rockers move equally as far as the motor turns over, because modern motor oil will destroy a 1968 flat tappet cam).
So in retrospect if you suspect a fuel issue substitute another fuel temporarily to see if fuel is the problem. Get a can of starting fluid and squirt as much as you dare into the car while cranking the engine over. If it starts up and runs you know you have no fuel in your carb or it is way out of adjustment. Rebuild the carb if you see gas shooting into the engine as you move the throttle arm by hand. No gas at the carb look for a clogged fuel filter or ruptured fuel pump membrane leaking gas into your motor (pull the dip stick and smell for gas).
If it will not fire with starting fluid you have an ignition problem. If you have spark to all eight cylinders consider pulling the distributor and installing it 180 out from where it is now as you are not getting the fire to the cylinders that need it (hence the chuffing and smoke as it turns over. Firing order on all Chevy's V-8 's (and Chrysler's to for that matter) is 1-8-4-3-6-5-7-2. Number one is the front cylinder on the driver's side and all are odd on the drivers side. The even numbered bank is the passenger side with the number 2 up front and number 8 in the back. Trace your plug wires to be sure they are going to the right cylinders. Also check your dwell angle if the points and condenser are not brand new; as burnt points and a shorted condenser will prevent it from firing up as well.
Finally consider twenty four volting your starter to get double the voltage to the starter motor (which spins it twice as fast) and doubles the voltage to the points as well for a hotter spark. If you have gas and ignition and 24 volts won't start it you need a new cam and lifters.