Metal Man gets an "A" for speeling.
OK - now for the REAL reason for warpage when blasting . . .
Sand blasting is a great way to remove coatings from any kind of material. In addition to removing coatings, another mechanism is at work that places the surface into compression.
A similar process that achieves the same type of compressive stresses is shot peening. This is often done to parts that will be placed in cyclical loading (compression/tension) like a piston rod. The shot peening places the outer surface of the rod in compression (thus placing the core metal in the rod in tension). This balance of forces prevents surface defects from forming a crack because the defect is kept in compression.
Any time you place a material in compression, you improve fatique life dramatically - that is why high tension cables are passed through concrete bridge structures to form pre-stressed beams.
Back to sandblasting - when the heavy sand media impacts the surface of the metal, it does not create enough heat to amount to squat. The real mechanism that is creating the warp is that the surface is placed into compression and therefore it will bow toward the sandblasting gun. This technique was used in the early days to assist in the formation of cowls and wheel pants on early aircraft except instead of using sand - they used very small steel shot.
SOooooo . . . in short - sand blasting works great to remove old paint and surface rust from stuff, but - it comes at the risk of metal deformation.
AD - you were lucky - OR - you are not as discerning as others in recognizing warped panels. My guess is that it is a combination of both. Either way, you have a great looking truck by the photos you have posted . . .
