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Hey Berserk, that is quite a challenge. When you say box, do you mean closing the open side of the C channel of the frame? I would think that inserting a smaller C channel inside the frame might add more rigidity. I would look to see where in the frame torquing is occurring and start there. You didn't mention what transmission you have. If Auto you should move to a TH400. What CID does your small block have? I've got a 89 C30 with a 454/TH400 HD package, factory tow package I might be talked out of.
Trying to beef up your 1/2 ton truck to 1 ton capacity will almost certainly cost you more in time and money than if you just bought a heavier truck. Not worth it in my opinion. There are parts of the U.S. where K30 trucks can still be found cheap. Craigslist and Facebook marketplace have deals all the time in my State. I would settle on a 1990s K30 before I tried to convert a square body. The 1990s models are priced cheaper than square bodies in much of the country right now. Take a one week vacation to rural Texas and drive home another truck.