If this was a fuel delivery issue, why wouldn't it start if I spray a little fuel directly into the carbs? I would imagine that would get the truck to run for 30 seconds or so..
It should start for a few seconds, Gig. I'm telling you something you already know, but there are only a few elements to worry about...gas (is it getting there?), spark (is it happening all the time and is it happening at the right time?), and valve timing (which if you have been running the truck already and if nothing blew up or made any sort of odd noise like a camshaft gear disintegrating should still be fine). Mike is right in that you can cover two of the carb intakes and plug up the fuel lines. But may as well go ahead and use the stock manifold since you have it.
As for spark happening all the time, I mention this because I've been stumped before by a failing ignition switch and also by a messed up wire going to the coil (bad connection which made contact off/on). You can't see these problems and sometimes you don't consider them. The wire in my case had broken inside above where it connected to the end which connected to the coil and I almost never found this. I believe it was still making contact but it wasn't a good contact any longer. The ignition switch had simply worn out. The contacts were making connection sometimes fine and other times very intermittently. I discovered this when I was able to wiggle the switch sometimes and get the thing to start. Have you contacted the prior owner to see what he did and how he "fixed" it? Pull your plugs when it won't start and see if any are wet with fuel. You'll know you're not getting spark consistently that way. You'll also know you're getting fuel.
