The hard to get stuff
Some where on here is a big long post I did on the conversion, it is not the be all end all on the conversion but pretty much hits all the high spots and some of the low spots. Did a search but could not find it maybe someone else can locate it for you and post a link to this thread.
The hard to find items are all Mustang II bits and you need them. The best find would be a manual shift MII engine complete from the bell housing to the front pulleys.
Note the MII engine and the Ranger/Bronco II version have some internal differences. The cam bearings on the MII version are smaller than the later 2.8, so the cams do not interchange. Also the main crank shaft thrust washer configuration is different on the early motors.
To fit the 2.8 in an Alpine, and it is a very tight fit we are talking 50 thou at some points, you need to have a 2.8 with a Ranger/Bronco II rear sump oil pan. The MII has a front sump pan and will not clear. So need the R/BII pan and pick up. These are easy to get. You also need a MII timing cover and the water pump pulley and crank pulley. The MII water pump is shorter than the R/BII pump and is required due to space limits. The R/BII water pump pulley will not mount on the MII water pump. The crank pulley has to be a single groove unit to clear the cross memeber. All the MII crank pulleys I have seen will unbolt down to a single pulley. Not sure on the R/BII crank pulleys but you many require the outer pulley be machined off. I have never tried a R/BII crank pulley so am not sure they will line up with the MII water pump pulley. You also need the MII lower radiator/heater hose water necks, which are different than the R/BII necks. You want the lower necks made of aluminium if you can find good used ones. On the back of the motor you need the MII bellhousing These bells only came on the MII and the first year Fox body 2.8 mustang which is a pretty rare car. When sourcing the bell you want to get the block plate if at all posible, thin metal piece that goes between the bell and the block. For teh fly wheel the MII unit is used but there is an isuue getting a clutch kit they seem to no longer be making them so it is a part you want to source early in the game if you can find one. Most parts houses list this clutch kit but when it comes to shipping it they just do not have one. The whole clutch thing is a sort of chicken and egg deal as in can you find a MII flywheel and can you find a MII clutch kit and which do you buy first.... and what happens if you find one and not the other. You can use a R/BII flywheel instead of the MII unit but there are a couple issues. The center of the R/BII F/W where it bolts to the crank is thicker than the MII F/W and you have to chuck it up in a lath and reduce the thickness to match the MII F/W in this area. The other thing is the R/BII clutch disc has the wrong spline count so you need to have a disc made with the correct hub, not a big deal.
The MII stuff is getting a little hard to find but it is out there you just have to dig for it. Buying the MII stuff piece meal can be a little pricey and the online junk yards do not typically want to deal with the small bits like the pulleys and block plate which is why a complete manual shift MII motor is usually a good buy even if you just steal the needed bits off it and put them on a R/BII engine. Oh and a 1974 non smog 2.8 heads are really rare.
Hope that helps get you pointed in the right direction, the only other thing I sugest to anyone building an Alpine is get the nicest car you can afford, solid rust free as posible and complete.