Boxed Rear Frame Rail

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Mpacker

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Does anyone know the growing pains associated with using boxed frame rails to replace OEM? My dad had some he left behind for the 70 Dart I'm trying to plan a build for. It looks like OEM are 3 1/2" tall, while these are 3". I'm not overly attached to them or anything, just wondering if it would be worth it to try and use them.

Would they be snug to the underbody / floor, or need to be floated down the 1/2" difference then have a spacer of some kind fill the gap between them and the floor?

Other "planned" mods to account for would be spring relocation & mini tub.

While proofreading that, I think I realized it would probably just be easier to get OEM replacements... I'll go ahead and post this anyways, as It was a little difficult to find substantive discussion on the topic.
 
I don't understand the need for this modification. The frame rails are already boxed.
 
The replacement rails he had were closed steel, 2x3” boxed steel. I called auto-weld (I believe he got them there) and they said they are sold for people who wish to narrow their frame rails to be able to fit very wide drag slicks. The frame rails on the Dart are rotten anyways, so I was seeing if I could save a couple hundred by using the ones he had.
 
The replacement rails he had were closed steel, 2x3” boxed steel. I called auto-weld (I believe he got them there) and they said they are sold for people who wish to narrow their frame rails to be able to fit very wide drag slicks. The frame rails on the Dart are rotten anyways, so I was seeing if I could save a couple hundred by using the ones he had.

Post some pictures.
 
The rears aren’t listed anymore, but same material.

BD74BFE4-DF6B-4031-B9DA-048559E0D52B.png
 
well, they're both. it's a C channel that's boxed by the floor
 
I guess I’ll sell them. Too much for me to try and figure out. Thanks for the input.
 
Rear rails and spring boxes on my '72 Swinger are rotted out the back 16". Found that stacking two pieces of 1.75x1.75 square tubing fit perfectly in the rails. Extended them all the way the where they kick up over the rear end. Just need to get someone with better skill than me to plug weld them into place.
 
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