All AMC spindles interchange. If you have an AMC car with disc brakes the entire setup will bolt off and bolt onto a drum brake upright. Just get everything from the caliper bracket out, including the spindle. Disc and drum brake spindles have different thickness bases, that's why the spindle is needed. In general dis spindles are thicker due to the different offset of the rotor/hub. You could space the drum spindle out if necessary, but most get the disc spindle.
The problem is parts availability. 79-83 AMC units are the ONLY ones I'd consider using. Some chain parts stores still have a source for the 79-83 rotors (calipers are easy to get for all), some don't. Last one I bought from Auto Zone for $55 two years ago -- Advance couldn't get one (AZ might not be able to now!). You have to go to an AMC parts vendor (yes, there are 3-4 out there!) to get the older rotors, maybe some antique brake parts vendors, and they are $100+ each. The Scarebird setup for $400 or less with current production parts is a lot better in the long run. If you need new rotors for a pre 79 AMC setup you've easily spent over half the cost of the Scarebird system. If you have a parts car with turnable rotors it might be worth using them, until you need rotors a few years down the road. Even the 79-83 should top $100 each in the next 3-5 years. The late model Scarebird parts will be available a lot longer, and right now they are using a $20 rotor. Buy a couple extra sets and store them!!
The AMC spindle on the 40 Ford upright with a Scarebird kit would be a reasonably priced disc brake upgrade. The only modification would be to the upright to the AMC spindle bolt pattern. Most AMC spindles are the same on the end, just different thickness to the base -- which can be spaced out or milled down. The exception is 75-78 models. Those used a big bearing spindle -- you want ANY 1952-83 Nash/Rambler/AMC drum or disc spindle EXCEPT 75-78. A Mustang II rotor will even fit the AMC spindle, but you'd have to make a caliper bracket to align the caliper over the rotor as the MII rotor has a different offset than the AMC rotor.
Speedway makes a 37-40 Ford disc brake kit that requires no modifications though -- and it's reasonably priced at $250. No point in spending that much on an AMC spindle conversion!