Don't let the engine light being on fool you. Chances are it's throwing other codes. In this case, I'd better dollars to donuts you're going to pull a 300 (multiple misfires) code or one if not more cylinder specific codes (301-306).
The 4.3 has a few issues to start looking for, the most notable a distributor with a bad bushing. Take the cap off and have someone turn the engine over. See if the distributor is turning true or wobbling.
Did you get the fuel filter tight? Stupid question, I know, but I have to ask to cover all bases. I repaired a 4.3 powered 1500 not too long ago where the owner had hacked in a smaller filter because he couldn't get the fittings loose and AZ said "this will work." Drip city. Once the filter was put in properly, the truck ran fine.
This truck should still have the ignition module. Sorry, I don't have the wiring schematics in front of me for the module to tell you what wires need what power or reistance, but you should be able to come across it online with a google search. If not I'll see what I can do to post it tonight. Same with the crank sensor.
Now, here's one I hoping you don't have: I can't remember the years but there's a TSB on the 4.3 (and the 5.0L and the 5.7L) that states the valve guides weren't machined right and the valve are sticking in the guides causing a 300 code. Not sure if it covers your truck. I can check. The basic check is to take the valve covers off, pop the springs off the valves, and move the valves up and down by hand while turning 'em to see if you feel any bind.
Good luck!
