TA cars didn't have firm feel just fast ratio. The faster ratio was due to a longer pitman arm. The P/S box you have, if really an original TA car box has the internal stops closer to work with the longer arm (this would be a bad thing on anything else). The firmer reaction spring showed up about 79 in the Diplomat/Grand Fury cop cars.
Any Mopar steering box will physically bolt to a 63-66 K-member, but I believe the 63-66 pitman arm was for the smaller sector shaft only. In any event you must use the 63-66 pitman arm.
Why would you want power steering? It's heavy and not necessary on an early A. The original Formula S cars came with 16:1 manual steering only, and if you read an original road test, it was praised.