Post-WWII and Pre-1976/1975 is around the timeline, but idk if it’s worth it… just buy a Toyota or Honda?
Lots of soldiers came back from the war and needed cars, so tons of investments were made into the ‘65-‘67 era Mustang and their competitors. You can still find enthusiast parts and some OEM stuff for mustangs. With that said, those were built hastily. It’s a bit of a crappy car imho. Lots of cars around this era kind of sucked compared to what they’re producing today in terms of safety and overall comfort. Around ‘70 is when emissions controls and some safety items were introduced, and ‘72 is when more emissions stuff and more safety stuff were introduced. Little things like an extra gas pipe to collect emissions, yellow side markers, and shoulder belts were standard by 1970. Around ‘72 and onward is when the chips were introduced. 1975 is when Jay Leno got the California board to relax on emissions controls for registration, so 1975 and earlier cars are exempt from smog; this doesn’t mean they didn’t start introducing chips.
Best bet is to find a 60-70s car with a nearby junkyard that has lots of parts to pull from. Online forums are a bit dicey to get parts from, but sometimes people have stuff you will need that they’re looking to part ways with. Online forums also have tons of info from car enthusiasts, and they’re really helpful. For example, how to convert your car to fuel injection.
Unless you can find a pristine running low mileage car, it’ll cost you nearly as much as buying a new muscle car in terms of everything you’ll need to replace to get it up to par with reliability and today’s gas. There’s no real way to get it up to par with safety, unless you put a roll cage, stiffen the suspension, weld subframes, convert to discs, get a five point harness, and etc.
Dodge/Mopar is the most expensive but boy is it reliable manufacturing. The problem is you’ll be hard pressed to find new American made parts nowadays and often the metals or sourced parts are from Russia or China. Esp with Covid/supply chain issues, the custom parts you need take 2-4 years to come… You’ll need to check for tolerances and slop on everything you order and pretty much have to have a very strong background in blueprinting cars. You’ll also run into problems when asking for help at the local Pep Boys/O’Reillys/Autozone counter. It’s as if your carburetor kit is missing float springs because everyone there hasn’t dealt with carbs since they were born and doesn’t know what those are. If you pick up those dumb manuals they have at the counter, sometimes it’s written in a different English where bonnets and spanners mean trunk and wrench - and some tools don’t even exist anymore because the new cars today automatically tension your s-belt when you need to pop in a new water pump, alternator, or whatever.