Using a modern (typically T-5) transmission requires minor widening of the Alpine transmission tunnel on the left side.
The WEIRD Alpine steering system causes a lot of interference issues with almost any engine except the original Rootes 4-banger. The Ford 2.8 V6 has an unusual exhaust port placement that happens to miss (barely) the Alpine steering box on one side and the pivot bracket on the other side. Jose Rodriguez (V6 Joe) has figured out the problems and solutions and sells a well designed, beautifully fabricated, rediculously inexpensive kit to put the Ford 2.8 V6 in the Alpine and provides pretty much unlimited support. One issue with the Ford 2.8 is that production of the Mustang-II version was stopped after 1978 and the Ranger version was stopped after 1984.
Other V6’s can be used if you are willing to tackle the engineering issues. I am in the process of putting a GM 3.4 V6 in my S-V. The GM engine is still in production, is 20% larger (with engines, bigger really is better) and was made in both carbureted and EFI versions.
I have seen pictures of an Alpine with a GM 3.8 V6 (90 degree layout), but there were some serious modifications required. The modern DOHC V6 engines are neat, but the heads are huge and they are mostly FWD (no RWD bellhousings and transmissions).