1963 Valiant - Rear differential Help please

This is as designed. The carrier is an is an interference fit into the differential housing, because the carrier bearings are tapered roller bearings require some preload. I "think" the spec is like .006-.008" preload, but it's in the service manual. Now, about how to do that. The "correct" way is to use one of these. It's called a case spreader.
https://www.amazon.com/Mophorn-Differential-Housing-Spreader-Axles/dp/B07RFMBBSH?tag=fabo03-20
This is the correct way for all Dana differentials and although no one ever says it, the 7 1/4 IS a Dana. It's not a 27, 30, 35, 44, 60 or any of the other bigger ones, and I'm sure it has an assigned number, but I've never seen it referred to. But it is a Dana.
Notice the two "nubs". They fit into the holes on either side of the differential carrier housing. When you turn the turnbuckle, it tightens against the case and spreads it apart. But don't go too far. Usually no more than .020" is needed. Install your carrier, the races and shims and then loosen the spreader. But, I've been working on cars since 1974 and I've never used one. I always just get the carrier and shims at a little angle and gently tap them into place. Admittedly, I have broken a couple of the thick, cast iron shims through the years, because they are brittle, so if you do it that way, be gentle.