A-Body Frame Rail Connector

res1vw21 said

while they may look clean installed think about it you are using a relatively weak floor as your top portion of your box, so you have a thicker U then a weak floor.......... I would use a nice thick wall box and make them yourself. Thought they are a lot better then nothing.

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My opinion is they are stronger than square tubing. No matter how strong the tubing is, bottom line is all the stress is being transmitted where it is welded to the car.

The USCartool ones are welded all around its perimeter, transmitting stress through a broader area so they wouldn't need to be as strong as ones just welded at the front and the back.

As far as the thin metal of the floor being used to box it in, I think that is very strong, because the floors tensile strength would have to fail (rip away from where the floor is spot welded to the top of the rockers). If it's strong enough for the rest of the frame just being spot welded to the body, then it would be even stronger fully welded.