Back in their day, 270s pulled 36' van trailers with a legal gross of 58,000. Pulling a box trailer, wind resistance will be just as hard to pull as a load. Especially with a side wind. I would consider a sway bar because 55 year old suspensions weren't as stable as what we have today. And good trailer brakes are a must. I would prefer the 4.57 because you will need the rpm's for power and to keep the engine running cool. I would also consider a thicker core radiator. Overheating was the biggest killer of GMC engines.
