a833 into a '63 Valiant

AFAIK, both the A230 and the A833 have the same bolt pattern, so should take the same bell.
Physically they are very similar. The output shafts can be the same.
As for the B-body trans not fitting, I think that is a mistake. You can grind off the front shifter mount for clearance, then use the rear mount, with a new home-built adapter-plate. This gives you a chance to put the shifter wherever you want to.
I did this on a 68 Barracuda, putting the shifter about 7 inches to the rear, between the buckets, next to my thigh, and jacked it up high enough to install the top shifter mounting bolt from inside the cab. Then I fabbed up some longer rods using stout tubing and pirated ends. Finally, I used a very short shift handle. It sounds like a lotta work, but only took me a few hours.
I agree with the Commando 3.09 low box being a better fit for a slanty, but second gear is then further away, so in compensation,you might have to rev first gear higher than what you are accustomed to, to drop into second in a better place. I don't think this will be too much of a bother. The 2.66low gear box has a tighter 1-2 split, otherwise they are ~same.
The long-tail will have a 727-sized slip-yoke on it but that don't bother you either on account of the driveshaft will have to be shortened anyway. This will be an extra cost.
The short-tail Commando will bolt right in.Only the Commando will bolt up to the 3-pot coupler.
The A230 is a short-tail, and will need a new driveshaft as well, different from any of the above.

IMO any 4 speed for a slanty is a good deal, because the ratios are much closer together than in a three-speed. You will have the right gear for the circumstances more often. A 5-speed would be even better.