I'm no expert, but I recently built my 64 fury with spindles from a 73 dart. They are interchangeable right to left, but putting them on so the calipers are to the front is the best for ease of brake line installation.
Now, if you have a sway bar, that has to be considered. If using an early bar (69-back) you have to mount the calipers to the rear to have clearance for endlinks. If using 70-up factory sway bar, then putting the calipers to the front makes it so you can just use OE flexible brake lines and route them as stock.
I've got 70-71 k-frame, lower control arms and sway bar on the fury, so calipers are mounted to the front.
BTW, I learned all that AFTER I had set them all up with the calipers to the rear. Was easy to see how the flex lines went after I switched them...