All I can say is check the rear end first. Like I said before, GM started putting the 8.5" 10-bolt in these cars in late '71-'72, BUT...............that doesn't always mean that the original rear end that GM put in the car is still the rear end that is in the car. Some idiot dillweed could have always changed it out for an 8.2" 10-bolt due to breakage or other problems. I bought one from a '72 Monte myself sight unseen for $100 , should have checked it cause it turned out to be a 8.2" 10-bolt. The 8.5" 10-bolt rear ends from early Chevelle/Malibu/Monte's around here have become very hard to find. Honestly I would say that you will probably find a 12-bolt from the early cars much easier than the 8.5" 10-bolt. I see them pretty regular in stock form on craigslist for $250-$300.
Another thing to consider, 8.5" 10-bolt rear ends are plentifull in junk yards/recycling yards (or your local U-pullit yard) for cheap,to fit mid 80's Full size GM Impala, Caprice, Parisienne, Lesabre, etc.... Yes it is way way too long, but if you are gonna build it anyways, cut the axle tubes down to the right length, install Ford ends onto the housing, and put in the right length axles of your choice. For another $100 Speedway disc caliper brackets and new Gbody calipers. Just a thought.