[james schaechter]

I had a 1985 Heavy 3/4 with 4.10s (2wd) and T-400 warmed up 350 and it towed an open like it wasn't there. I didn't get great mileage, but it didn't change much towing or not. If you have a deal on a 4.10, I would go for it. If you start buying stuff new parts , it gets pretty $$$. I am guessing you have a 4 speed with granny low then.
If you start changing a bunch of things, you might be better off looking for a different truck. I have had several stick shift 4x4 3/4 tons (mid 90s GM). You can find them priced right. My brother has my old 1997 truck and it is awesome. It is an ext cab vortec 350 5 speed with 3.73s. It still pulls our 24ft enclosed at times and pulls it no problem. It has over 200k on it. Original engine and we just replaced the clutch. If yo usold your truck and bought one of these, I bet it would only cost a couple thou different. I would tow in 4th at 55 mph and 5th at 65 mph or above.Make sure you look for the heavy 2500 so it has the good HD trans. the med duty one has a small clutch and an integral bellhousing. They also ride nicer with the independent front susp instead of the straight axle. It might be worth getting a calc out the $$$ and do some research. you might find some deals.
By the way, many of the 1/2 ton SUV types like a Yukon for example have the power. I have a friend who towed a 26 ft enclosed with one when we he first got back into racing. I was expecting probelms with braking or suspension. With load bars, it handled well, stopped well (4 wheel discs). the weak point was actually the pinion bearing. Not strong enough for the weight on a long tow and he knocked 2 of them out before he upgraded to a 3/4 ton. JIm.