The way I understand it, if the car is all together and someone wants to add these types of reinforcements, they need to be added with the car resting on its tires with the suspension under a load. If the car is suspended with tires hanging, everything sags a little bit. If you weld in connectors and torque boxes then, you may have affected the body gaps, front wheel alignment and drive shaft pinion angle.
The cars on rotisseries have no engine/trans/suspension, axle, steering, so they will not be affected.
The last car that I installed frame connectors was a 71 Road Runner. I used a 2 post lift to raise the car, then I placed tall jackstands under the rear axle and lower control arms to simulate a condition of the car being on its tires. The car had the same body gaps before and after.