Joined: Dec 2007
I have installed a 100amp large case alt. from a mid 70's full size LTD, Lincoln, etc. It fit perfectly with my stock mounts and just needed a 1/2" longer belt I believe it was. It uses the exact same VR as the stock mustang 43A-62A. I bought the wire plug to go with the alt and just matched the wires up and crimped them in. It's easy to match the wires, they're labeled on each alt. by +,-, stator and field.
I also plan A/C, stereo w/amp, 55W driving lights, rear window defroster, etc. so I needed the extra amps. I haven't added all that yet but it works perfectly so far.
I know of no parts store that you can walk into and say, 'I have a 63A stock alt., please give me the next biggest' 63A is as big as that stock casing ever came. You will find 100A alt. built into the 63A casing on ebaay but it is too small and will overheat and burn out in short order. I have spoken with 3 private alternator rebuilders and they have all stated this, the casing is simply not big enough to extract all that heat or have large enough heat sinks.
You can also rewire to remove the VR and install a later model 1 wire alt. (120A and up usually), search the web for this, it has been done many times with instructions.
I find that parts store information and service always depends on the person behind the counter. I primarily use O'Reilly's since the counter people are more helpful and they can get name brand parts I want like Raybestos brakes same day/next day for no charge and if it's not what I want they send it back no charge.