Headers for a A body

Professor Fate said
They can be made to fit, but like I said, you have to be willing/able to do fab work to modify them. Remember, these are designed for later ('67 and up) A bodies, the early As are quite a bit more confined.
The passenger side fits fine, however the driver's side needs a bit of work.
Stock:
View attachment 1716190275
Modified:
View attachment 1716190276
Note that the #1 tube has to be cut and rerouted so it joins the #3 tube, and the collector needs to be reworked after #1 has been removed from it. Since #1 and #3 don't fire at the same time, sharing the #3 exhaust tube has minimal effect on the performance of the shorty header.
63 dart v8 swap

For this I would get a copy of
Scientific Design of Exhaust and Intake Systems. Written by Philip H Smith and John C Morrison. Fair technical. One header manufacturer read it and marketed a header using a described technique which relocated on primary tube per side into the collector instead of up stream of it. This modification may work for fitment and considering 1 and 3 fire 270° apart should not harm power, but not aid it.