I had a similar issue with my 55 1st series a few months ago. A friend of mine, old time mechanic, did a carb rebuild for me. Afterwards it would die at low idle and sputter just above idle. It only really ran at higher rpms. What we found is that the chemical cleaner during the rebuild knocked some build up loose, but didn't remove it and clogged up the carb so it only worked at high rpm when gas was being pulled through the carb a lot harder. Oddly enough we just tapped the side of the carb a few times with a wrench, firmly but not violently, and it worked out whatever was clogging the system.
Also, I suppose if you lowered the floats too much that could starve your engine, but I would imagine that would be a problem at higher operating speeds, not deceleration.
I can't claim much if any knowledge on your year of truck, but if you have a mechanical fuel pump in there, that could be failing to work, and since that does depend on rpms, it might also be the problem.
I hope this helps, I'm not genius, so take everything I say with a grain or two of salt.
Good luck ApacheFiend.
