This is a front t-box install just so there is no confusion in case you are doing rears

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I used a 67 box. The t-box pictured is laying as it installs in the car. The top part you see is what faces the floor. You can see the jut out where it overlaps the lower sub frame sitting on the floor and the other side faces the tire. The box is designed to slide up between the sub frame and outer rocker, there is nothing that 'lips' over making the upper demension wider than what the factory clearance should be.
http://www.dodgestang.com/images/box.jpg
It was inserted from the bottom. Start the box slightly forward of final resting position (like 1/4 inch since there is the front frame rail lip you are also trying to sneek past) so the front weld lip does not catch on the way up, once it is past the point were it would catch slam it back to the 2 lips are touching. Then you can wail away to get the box all the way up...it is not easy, and the fact that the car is 40 yrs old and the clearances may have shifted, doesn't make it any easier.
http://www.dodgestang.com/images/dsboxinstall6.jpg
The upper part of the box that goes against the floor one side has a lip that is bent at 90 degrees to it will sit Parrallel to the outer rocker not lip over it. This rests against the outer rocker just north of the floor, so we had to cut the floor here:
http://www.dodgestang.com/images/DSboxinstall2.jpg
Right were the up down meets the floor board.
In this picture you can see the bent lip poking through the floor board and the weld:
http://www.dodgestang.com/images/dsboxinstall8.jpg
The t-box is a VERY tight fit. The PO on my car has welded all kind of crazy metal on the car to stiffen the subframe which had to be removed before we could even lip the box lightly into place. We also used the grinder to remove about 1/16 of inch in 1 spot to get it up and in easier. The t-box in a very important part, and IMO becasue of the way it is designed not something you want to pull apart and reassemble in place. That defeats the purpose. Because we were able to get the t-boxes with only using the wieght of the car as leverage to squeeze it into the space, we
KNOW the car is pretty darn true and straight. If you pull the box apart and then reammeble you don't know that and in all honesty may just be stiffining up something that might need to actually be repaired.