Engine needs 3 things to run - 1. compression 2. fuel 3. ignition
1. This should have been the very first thing you checked when you first got the truck. As stated before, check all cylinders. If compression is down in any cylinder, stop. Engine has internal problems. If compression is down, squirt a bit of oil into the cylinder and retest. If compression improves you need rings, if it doesn't help, you need valve work.
3. If the thing ran before you mucked with it, all you need to do is check for spark at ALL cylinders. Timing hasn't changed and should be fine. If all cylinders are sparking in the correct order we move on to fuel.
2. Since this is what you were mucking with, it is the most likely source of the problem. You have already tried dumping a bit of gas in with no fire. This leaves only a few possibilities. They include a non functional carb (unlikely as you tried the fuel dump), no fuel delivery to carb (again unlikely due to dump trick), or a vacuum leak. Are all intake external holes plugged? are all gaskets covering what they are supposed to? is the intake you are using a good fit for the engine? were all gasket surfaces cleaned shiney with acetone before assembly? etc. gummed up carb? gummed up fuel filter?
The one other thing that comes to mind is temperature. You say that the truck is cold. Was it running well in the summer and got worse as the temperature dropped? What weight is your oil? A heavy oil works great in the summer, but when it gets cold it can slow the speed the engine cranks over and will not start easily. Mine had this problem up here in Canada, a lighter weight oil in the winter perked her right back up again.
The slow decay of performance is a bit puzzling. Also don't forget this truck has been parked for 15 years. An internal problem may have been the reason it was parked in the first place. Start with a compression check, it can be revealling.
good luck, and keep us posted.
Ryan
