I looked up what I refered to in my earlier post and thought I'd post it. The "1965-66 Ford F-100 F250 Illustrated Facts and Features Manual" says that both the F100 and F250 have 36,000 pound per square inch yeild strength steel in the frame, though I would assume the 250 had a greater cross section which may be what this next number relates to: Section Modulus for the F100 was 2.98; for the F250 was 3.71; and for the F250 4X4 was 4.74--there, apparently, is the real strength difference for the heavier trucks. Any engineers out there, please explain section modulus to me? I work with a bunch of civil engineers who apparently don't know. Need a mechanical engineer.