Ok first take deep breaths.Sadly old cars are Like old: houses,people, bread, etc.LOL The harder you look the more issues you are going to find.So at some point you have to stop looking,probing,digging deeper into the project.Until you get a handle on stuff. Seriously I say this because it is ridiculously common for people to jump into the 10 foot water and drown in a bottomless sea, Yes these cars can be overwhelming like almost any project right. I'd say set a goal of getting the car running and inspected if that's really necessary at this point?I assume this is not your daily driver,correct? Then that should take a huge load off of your shoulders.
Then get the heat squared away and then start opening new cans of worms. As far as your heater box, I'd like to see photos. Then I can advise better They would help me help you.I would also say don't throw anything out yet not matter how bad it looks except for the actual heater core. Yes the repo boxes look good but I'm also under the impression That they do not contain all of the guts? Nor do I believe all of the guts are repoed yet. So we'll take a step back and see what exact parts you need. Testing your heater? First check your blower motor, Supply power to it and see if the motor turns,if you wire it directly it will only work on one speed,High.If that works you're one step closer.
Ok Mustang Books and Magazines. You can usually find the Mustang magazines at places like Barnes and Noble. As I said there are really only two at this point that might benefit you I would say Mustang monthly and Mustang Magazine. Mustang Magazine is a start up ,they have had a lot of hold ups especially in terms of delivery ,which I guess can be expected for a start up. I'v etalked with them numerous times and I am a huge supporter of the magazine. That said if you go onto their website
https://mustangmagazineonline.com/ You can sign up for their Free test drive,It says a one year subscription which is a glich,but when you sign up for this you get a free issue mailed to you. They are currently doing subscriptions for $20 per year. So there you go my free shameless plug for a new magazine. For the record I'm not affiliated with them at all,Just a classic Mustang magazine geek.
Very often I 'll buy my books and magazines through places like ebay, I usually get good used items for fractions of the new cover prices.As I said before the best classic Mustang magazines and books were the ones from Mustang Monthly from the 80's and 90's,In my opinion. P.S. If you get interested in accummulating a library ,let me know? I have some extras kicking around that I've accumulated whie trying to complete my Mustang library,although I have sold and traded a ton of them off already..... Anyhow I'm going to rest my fingers now before I reply to your inspection question.....LOL