Well this is what I can help you out on. I have an 8" from a Mustang II and it bolted right in. I think it was a 77. I put a posi in the housing that was from a late 60s mustang. All 8" centers will interchange. It will be VERY hard to find a Mustang II that already has a posi in it. Although some did come with them. I have an 88 turbocoupe motor in my wagon and it still has the 5 speed behind it. I don't see any reason why you couldn't bolt a 5 speed behind your stock pinto 2.3. I did have to make a plate that goes between the trans mount and crossmember because the bolts did not line up. That is about it. If you reuse your clutch and bellhousing, things should be a little easier. You will however, have to cut your drive shaft.
As for rear disc upgrade..I know you can also get these from the 87-88 turbocoupe. I've seen people do this but have not done it myself, so I don't really know how it is done.
Right now I have 14x6 rims on my car with no troubles. If you try to put on 85-93 Mustang 15s, they will hit when you turn because of the difference in back spacing.
Hope this helps.
SVOwagon