How fast are you trying to go? I have pulled a lot heavier loads with rigs similar to yours. I did not have much trouble with any of the pickups. Down shift it manually and give it hell! Now a warmed up 350 will pull a lot better than a Target 350. Have you checked the compression of your engine. She might be tired. The problem I have found is we get spoiled by the power and driveability of the new stuff and forget how to make the older stuff work. My buddy used to pull a 20 foot stock trailer loaded down with fat cattle with a 260 straight six all the time. He would all ways say "any one can pull with power, only a real man can pull with a six cylinder". A manual four speed would be a lot better too.