How do you free up frozen body bolts on an old International without snapping them?

Heat is ussually the answer for the stubborn ones, but you're right to be careful with it on that truck. I did a '37 D-2 a few years back with the exact same setup, bolts up through the frame rails into wood sills. What worked for me was a small torch tip and heating just the bolt head or nut for maybe twenty or thirty seconds, then hitting it with the breaker bar while it's still hot. You're not trying to get everything glowing, just enough expansion to crack the rust bond in the threads. Keep a spray bottle of water and a fire extinguisher right there, and put a wet rag or a piece of sheet metal between the torch and any wood. The old paint will bubble and stink but that's cosmetic at this point.

If you'd rather not run a torch near the wood at all, try the candle trick. Warm the bolt head with a heat gun if you've got one, then touch paraffin or even a regular candle to the joint and let it wick into the threads as it melts. Works better than most of the store-bought penetrants on seized fasteners. Also, give the ones you've allready soaked another day or two. Penetrating oil needs way more time than a week on fifty-year-old corrosion, and reapplying it helps.

Another thing that saved me: an air hammer with a blunt chisel bit, striking the bolt head square on. The shock alone breaks loose bolts that a breaker bar just twists. If you don't have an air hammer, even a big dead-blow hammer smacking the head a few times before you pull on it makes a difference. On the mounts, don't count on repro parts for a truck that age. Some of the IH suppliers have sheet metal and hardware for the late 30s trucks but body mount plates are hit or miss. Honestly, fabricating new ones is the way to go. The originals are simple flat stock with a hole in them, so trace one intact-ish piece onto new plate steel, cut it, drill it, done. If every last one is rust flakes, measure the bolt spacing and hole diameter off the frame and box and build from there. And before you lift the box off, photograph everything and label the bolt locations. You'll thank yourself when you're putting it back togehter next year.

On my old '41 flatbed I quit trying to turn half of them and just ground the heads off. If you're replacing every bolt anyway, which you should be after fifty years, a seized one has nothing left to give you. Cut the head off square, punch the stub up through the hole, done in two minutes instead of fighting it all afternoon. I used a die grinder with a cutoff wheel but a hacksaw works if you've got patience, that's how my old man did his ladder truck back in '68.

For the mounts, flat bar from the steel yard is cheap and plenty strong. I bent mine cold in a big vise with a length of pipe over the handle, no torch needed for plate that thin. Makes a nice afternoon project while the penetrating oil keeps working.

Gonna push back on the 'punch the stub up through' part. On bolts that run up through a frame rail into wood, that stub is usually seized in the hole itself, not just the head rusted on. I fought a similar setup on an old grain truck and half those studs wouldn't budge either direction because the shank was rust-welded to the plate. Ended up drilling them from the underside anyway, which is exactly what the OP is trying to avoid. Grinding heads works fine when the bolt can actually slide, but nobody should assume it will here. Might be worth testing one before committing to that route on all of them.

I'm actually working on an older cab chassis, so I don't have the box setup, but I'm running into similar rust issues where the frame connects to the cab structure. The bolts themselves aren't the main problem, it's more about the sheet metal around the mounts being completely brittle and flaking away when you try to get leverage. I'm trying to figure out the least invasive way to cut out the rusted sections without damaging the surrounding steel too much. Has anyone found a specific method for removing deeply rusted structural mounts when the metal itself is compromised? I'm worried about cracking the frame rail if I try to cut a piece out to replace it.

Update: heat got a few more of the nuts spinning, so that part's moving along. But the grain truck guy was right about the stubs, a couple of them won't budge up or down even with the heads off. They feel welded into the frame holes. Anyone had luck drilling these out from underneath, or is it better to just drive them down through the rail with a big hammer and call it done?