No power steering 915 casting heads
Okay, with manual steering you have options.
I'llonly touch on the driver's side manifolds, since the passenger side ones all seem to fit okay, only the port size varies.
The original 273 A body manifolds ('64-'66) will fit even with power steering, but the port size is smaller than your 915 "J" heads, and they are fairly restrictive especially for a modified motor. They do not fit (without modifications) on later heads that have a larger exhaust port surface, but your J heads should be fine.
340 manifolds will fit, as RRR mentioned, but may require shimming a motor mount or a bit of grinding to get optimum clearance around the steering column and coupler. Won't work with power steering, no room. They are also very pricey compared to other options. Almost the cost of good headers.
(Pic stolen from 65 Cuda 340)
Magnum manifolds will fit with manual steering, but not power. Earlier ones have a 2 1/8 outlet, later ones are closer to 2". They flow very well even though the ports are smaller than the ports on your J heads. They also require more work to get them to fit, grinding the outside for column clearance and shimming the motor mounts. Some people have even had the mating surface ground or milled a bit in order to gain more clearance. You'll most likely need to trim back the steering column collar and reposition the steering column as far to the left as you can, every little bit helps.
YES 5.9 Magnum manifolds work in early A bodies. Here is exactly how!
As far as dual exhaust goes, TTi makes headpipes that fit the stock exhaust manifolds.
Exhaust_Parts
You're on your own (Muffler Shop) with the 340 or Magnum manifolds.
And as far as installing duals, the transmission crossmember needs to be modified for duals, sinc these cars were designed with only single exhaust in mind.
TTi has instructions how to do it, or there are other places (Layson's comes to mind) that offer modified crossmembers, but you may have to supply them with a core.
https://www.ttiexhaust.com/Sheets/Crossmember_sheet506.pdf