The guy I was working with made ladder frame sections 8 foot long. At the rear it was 18 inches tall and the top tubes were curved down to 12 inches up front. As said all the diag's were 5/8x0.063 DOM. He then placed the single ladder on blocks and set a dial indicator under and loaded 300 pounds in the center to measure deflection. From memory, the difference between 0.120" and 0.085 was about 0.25 inch. In a bounce, he lifted up on the center and dropped it 4 times, I think it was like 7/16 inch difference.
But in these tests he did not do a 100% full weld on the diag's so there may have been some loss of rigidness.
He ended up using 2 of the 1.5x0.085" tubes on the bottom and 2 of the 1.5x0.063" tubes on top.

"This thread is worthless without Pic's" Yes I lost all those pictures when a hard drive failed. I'll see if he has pictures on the Web of the build.