Your truck will struggle with that load with the old V6 and 4 speed. How did it feel driving it home or have you not hooked the trailer to it yet ?
Looking at older trucks, I'd go minimum with the big V8 half tons. 5.3 Chevy, 5.4 Ford, or 5.7 Hemi. If you can swing it and get into a new enough truck with a 6 speed trans (around 2010) or especially a 5.0 V8 2011 or newer Ford, it'll be a more pleasant towing experience.
Shoot for as low of gears as you can find. 3.73s for most trucks, can get 3.92s I believe most years in the Dodges. That will help too.
Keep your eyes peeled for the right deal and don't discount 3/4 ton gassers either. Won't have more power if you get V8s but will handle everything even better.
