Do you have fuel in the bowl sight window? If not, you may have a stuck float valve, very common when they sit a while without running. Take off the input fitting, clean the filter and squirt some carb cleaner down into the inlet, that may loosen it up if it's stuck. If that won't do it, you may have to pull the cover off and take a look inside the carb, there may be deposits clogging the float, valve, or fuel jet inlets.
Will it start with starting fluid or a little gas poured down into the carb?
Ok, well, try pouring just a little gas down the carb, (it won't take much) then try starting it. If it fires then, you probably have sediments in the bottom of the bowl which are clogging the jets. If it doesn't fire doing that, it's time to check the compression on the cylinders and make sure the ignition and cam timing is correct.
One thing to keep in mind, if a vehicle sits for a very long time, ground squirrels, rats and mice can use the exhaust system as a place to make nests and store food. If they have the exhaust completely blocked, no amount of fuel and spark is going to help, because the engine can't get any air. Sometimes if you can get the engine running, it will run very rough. I've had some strange things come flying out of an exhaust on a vehicle I had sitting for several years.
So, considering you know you have spark, if it won't run when you add fuel to the carb but your compression looks good, the ignition timing and cam timing look good, try disconnecting the exhaust at the manifold.
I forgot to mention, I had a 720 that I bought used, and couldn't get it started to save my life. I went through and fixed all the obvious problems, wiring issues, corroded connectors, bad fuel pump, fuel pump relay, stuck float valve, bad ignition coil, blocked jets in the carb, and after all that, it still just wouldn't start even though I knew I had fuel, spark and air. I checked the timing, compression checked the cylinders, changed plugs, and everything was good. But, if I poured a little gas down the carb, it would start right up, run a short while, then die...
In the end what fixed it was draining all the gas from the tank, and putting in fresh gas. This stuff was so old and oxidized, I couldn't get it to burn even if I poured it on the rag and tried lighting it with a lighter. So, don't forget the basics like I did!
Well, I'd check compression before I'd pull the head. Even with a blown head gasket, it should still be trying to fire on the cylinders that do have compression. If you have no compression on any cylinders, it might be someone put in a cam and didn't time it correctly, best case there is you just have to align it properly, worse case it bent the valves from opening them at TDC and you'll have to have the head worked.
Have you moved the ignition timing around to see if it will try to start? If the fuel pump shaft wasn't installed properly, the ignition timing could be way off.
What type plugs are in it? Are the plugs wet all the time, or just when you poured fuel in the carb? If they're wet all the time, you might have a float valve stuck open an it's fouling out the plugs. I've had plugs fouled with gas that would never fire again even after being cleaned and dried. Had to replace them.
Just keep checking off the basics, you'll figure it out.
Hmm, you have four numbered notches on the cam? Normally, there are only 3 on a stock cam gear. It sounds as if you might have a motorsports cam sprocket on that. It has 4 numbers, and 3 letters.
Those are for setting up the cam timing when you install the chain, you can't really use those to check the valve timing accurately with those once the engine's been spun. And, you have to have the front cover off to set up the cam timing using those marks because there is a relationship between the bright links on the timing chain and the marks on cam and crank sprockets.
You can check basic cam/ignition timing with just the valve cover off. Just pull the plugs on one side to make it easier to turn the engine over with a socket on the crank pulley bolt. Its good to have a friend help with this. Then turn it over clockwise until #1 piston comes up to TDC on the power stroke (you can use a phillips screwdriver down in the spark plug hole to determine when the piston is coming up to TDC if you can't read the crank pulley timing mark, just don't leave the screwdriver in the spark plug hole and spin the engine over unattended or you'll damage things), both valves should be closed (up) at TDC on the power stroke, and your distributor should then be pointing to the #1 plug position under the distributor cap. At that point, the numbered notch on the cam that is at about the 2 o'clock position is most likely the one it was aligned to. Normally this is 2 on a stock engine (from my experience). Each number represents a 3-4 degree of cam timing advance or retard.
It's very common with people who haven't worked with these engines much to get the ignition timing off when they remove and replace the oil pump. You have to install the oil pump shaft in a very specific way in order to insure your distributor timing is correct because that shaft also drives the distributor. This is somewhat difficult to do with the engine in the truck, more so on some years than others. There a some good discussions in the forums on this topic, you can try searching for "oil pump shaft".