'71-'73 Dart Tail Light Housing ?s

So he's deleting the reverse lights and just running two brake/tail lights? Ie, he's not going to modify the bumper and run 3 lenses across or anything like that. What's he going to do for reverse lights?

The housings are a zinc based pot metal, welding them isn't really advisable. You could probably braze them with the right stick as the temperature should be low enough. An epoxy would probably be a better bet though. With proper prep you could probably just JB weld them together. You could epoxy the flat section that faces the outside, then use some flat bar stock and some screws to reinforce the back side across the stock horizontal reinforcing ribs.

Not sure about the clearance, you can see here that the brake/tail section has a different light location, so there's that extension that sticks down.
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Honestly though, he might just try changing out the lens and the wiring and see how the light pattern looks. The reverse light section's reflector isn't as wide, so, it wouldn't light the lens exactly the same, but it may not be a super noticeable difference from the outside. As in, maybe not enough of a difference to justify all the work needed to modify the housings. Just a thought.