Boxed Rear Frame Rail

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.