I had responded to your post so I'll try to recall what I typed.
Your Problem sounds like you may have a timing issue. Set your base timing to 16° and see if it helps clear up the power and backfiring issues. In the photo, your plug wires look well worn - recommend you replace them.
The 600 cfm Holley is on the small side for a 351C 4v. It will run okay but you're giving away some performance.
The Edelbrock dual plane intake manifold you bought will work fine with your engine and the ram air assembly. You shouldn't have any clearance issues. I run the same intake on my 72 with ram air.
1. I think
@giantpune recommended the Pertronix 3 module, which is a popular option. I prefer to retrofit the Ford Duraspark 1 electronic ignition. It is a fairly straightforward procedure and if you use the standard Ford small distributor cap you will not have any interference with your air cleaner. Check out
https://pantera.infopop.cc/topic/parts-needed-to-convert-duraspark and
https://pantera.infopop.cc/topic/sticky-3-351c-basics-and-performance-tuning
2.
@Hemikiller found this video for us:
If you do change the camshaft, check with some of the custom cam grinders, e.g., Brent Lykins
https://lykinsmotorsports.com/ so you can get a cam matched to your overall build.
3. Your #3 steps all are sound.
I recommend you get either ARP bolts for your intake manifold from one of the speed shops, or buy all your engine's external fasteners from AMK Products
https://www.amkproducts.com/i-kits/ford-individual-fastener-kits/?year_=1971&model=Mustang&category=Engine - 351C
Check all your fluids, belts, hoses (heating/cooling and vacuum), etc.
I'm sure others will add more for you. Keep us posted on your progress!