The answer depends more on the strength of the corrugated box you've used in relation to the per-carton weight than anything else. A 12 inch high stack of records weighs quite a bit, probably ~30 pounds. A single-wall 200 lb. test box would almost certainly split at the seam at that load with any serious jolt. A 275 lb. test double-wall box would not. That information can be found in the roundel stamped onto the bottom flap of the box.
As far as stacking goes, I would strongly recommend against it. Far better for you to have the boxes loaded so that the records inside are perpendicular to the floor of the truck, firmly strapped or blocked into place so they can't shift and loaded in such a way that nothing can fall on or crush them from above.
On temperature, so long as the inside of the truck doesn't get above 100 F and you allow the records to return to room temperature for 24 hours before unpacking, you're good. I've moved my collection across the country 5 times that way without a single problem of any kind.
Good luck with the move and happy listening!
As far as stacking goes, I would strongly recommend against it. Far better for you to have the boxes loaded so that the records inside are perpendicular to the floor of the truck, firmly strapped or blocked into place so they can't shift and loaded in such a way that nothing can fall on or crush them from above.
On temperature, so long as the inside of the truck doesn't get above 100 F and you allow the records to return to room temperature for 24 hours before unpacking, you're good. I've moved my collection across the country 5 times that way without a single problem of any kind.
Good luck with the move and happy listening!