Don't stress this. I just moved 10,000+ LPs from NW Montana to Washington state's Olympic Peninsula. Two U-haul trips of two days each time. Middle of summer with temps routinely > 90F. I used the biggest (non-refrigerated) truck each time and U-haul "Small" boxes. Each box about 1.5 cubic feet. Each box weighed slightly more than 65 pounds.
The U-haul boxes were inexpensive, readily available at larger U-haul locations, and much stronger than some moving boxes I had on hand from Home Depot and Allied Van Lines.
I stacked the LP boxes 4 high and kept them toward the front of the cargo area and centered on the centerline of the truck, with lighter boxes filling out the area from the LP boxes to the walls of the cargo area. Lighter, bigger boxes also got stacked on top of the LP boxes all the way to the ceiling.
The 26' U-haul trucks ride very rough on rough roads, and I can't imagine roads much rougher than the construction area through I-90s Snoqualmie Pass.
Absolutely no damage to any of the records I've unpacked so far. I did all the packing, loading and unloading myself. Some tips:
You want all the contents of your truck tight -- everything snugly packed in side-to-side, top-to-bottom, and front to back. If you leave space between boxes the load will shift around and boxes could split if allowed to move and bounce. Build your load vertically to the ceiling before you start filling the truck from front to back. Every couple of vertical rows use some line or cargo straps to secure the layers. Most rental trucks designed for cargo have rails or wood rub strips on the side walls to secure a line or straps going side to side to secure the load. Imagine how the load will want to move if you have to slam on the brakes, or going through tight turns and pack accordingly.
Make sure you pack each box full. Loosely packing the box can allow the records to move and vibrate inside the box and can lead to ring-wear on the covers. Not having the boxes snugly full will also compromise the strength of the box -- that's why boxes crumple and fail. Absolutely a box will crumple and fail if it is not packed all the way to the top where the flaps fold over and other heavy boxes are placed on top.
Leaving the boxes filled too loosely could also allow LPs to warp, though I doubt it's going to get hot enough long enough to warp any records on your move. Cardboard is an excellent insulator, as are the rest of the contents of the truck. I had zero warpage.
For valuable LPs consider removing the record in its inner sleeve and placing it beside the record jacket with both jacket and LP inside a plastic outer sleeve. This will avoid seam-splits, though I have to say I haven't found any seam-splits in any of the jackets of the LPs I just moved, and the vast majority of them were packed with the LP in its inner sleeve inside the LP outer jacket or cover just like they come from the pressing plant.
Use a dolly to save your back and save time. If you're using boxes smaller than the U-haul "Small" box -- it sounds as if you're using what U-haul calls a "Book" box -- you can easily move four boxes stacked on a dolly or hand-truck. I didn't go more than three of the larger boxes at a time (approximately 195 pounds).
This wasn't the first time I've had to pack up and move house with all my stuff including a large number of LPs. When I was working most of the moves were paid for by my employer. Now that I'm retired it was up to me to move them this last time. Whether moved by me or by a commercial moving company -- and some were good, bad and one was really bad -- I have not had an LP damaged by the moving or storage process.
Hope this helps,
Steve Z
