Welcome from Pittsford, NY (Rochester suburb! We have two 1973 Mustangs. One is a deeply re$tored and moderately hopped up Mach 1 sporting a street/strip built 351W (not C), a 3.5:1 TractionLok rear axle gear set and an AOD 4 speed automatic tranny. The other is a True Survivor Convertible that was barn stored over 40 years. It currently has just over 20,000 original miles on it, and is in near perfect, beautifully preserved condition. Both have had Classic aftermarket Air Conditioning kits added (neither had factory A/C), Dakota VHX analog gauge instrument panels, Garmin Drive 51 with backup cameras installed, and alternators rebuilt with high amperage output components to produce 65 amps with the vert, and 90 amps on the Mach 1 with its first generation side terminal style alternator. The higher output rebuilt alternators were deemed needed as the original alternators were only 42 - 43 amp units, and we also added dual electric fan cooling to both. At best the original output levels were marginally adequate, and at that only if the engine RPM was running at 2,000 or higher. The boosted output provided good output even at idle speeds, ad more than enough output at higher RPM levels.
Both Mustangs are a blast to drive, and while the Mach 1 is pretty zippy in terms of performance, and very nice to cruise in with the AOD, we also have a 1969 Shelby GT500 and a 2020 Shelby GT500 that can definitely eat the Mach 1's lunch quite handily were we into such pastimes. Luckily, at 67, I have largely outgrown petty street grudge racing, and now just enjoy cruising comfortably with whatever pony car I happen to have decided to go out in. All of them have A/C, both Shelbys have factory A/C, which for the 1969 GT500 was and is a bit of a rarity. It was one of the reasons I purchased that particular Shelby.
Anyway, I hope to see some photos of your Mustang, and to see you pitching in with questions and/or suggestions to questions from others once in a while. That said, I do not know if you have adequate shop manuals and wiring/vacuum schematics or not. My main GoTo for such documents in PDF file format (and inexpensive) is Forel Publications. I will provide their link below. I have found the 1973 Shop Manual from Forel, Volume 6 finally provides decent engine vacuum system schematics, but for pre-1973 such schematics were hard to find - until I came across the schematics produced by The Mustang Barn (free). With their knowledge and approval I pulled all of their individual 1967 - 1972 Mustang/Cougar vacuum schematics into a single PDF file, grouped by year and sorted by engine. I make that consolidated file available to anyone who may need it on a no fee basis (also), for love of the hobby. It is attached to this response for you and anyone else who may need it do download for your own use and to share with others as you deem appropriate.
Also, please be advised that the Color Electrical Schematic manuals from Forel Publications for 1972 and 1973 are really good and very complete and accurate with very few minor errors, the 1971 and prior schematics are lacking in the kind of content depth as found with 1972 and 1973. Thus, even though you have a 1971 Mustang, it would behoove you to get a 1972, or better yet a 1973, electrical schematic manual in addition to the 1971 electrical schematic manual. There are very few differences between 1971, 1972, and 1973 electrical systems. The biggest differences I have found are that beginning in 1972 Circuit 640 is used to power the carburetor Throttle Position Solenoid (TPS) on engines that included the TPS, and 1973 the electric choke was introduced, where the alternator stator circuit (with 1/2 the voltage output in Alternating Current (AC)) is used to power the carburetor electric choke. Everything is pretty much identical over those 3 years.
Here is the Forel Publications link:
https://www.forelpublishing.com/clickbank/index.html#Top
