Autozone usually has replacement carburetors. Just tell them what the carburetor came off of and if you can take it off and carry it up there. They will take your old one as a core for swapping out. They will even trade you the carburetor you need for any carburetor you carry up there as a core.
Those carbs are very common. I had a 62 Econoline van with a 200ci six. The 144ci 170ci and 200ci engines used the same engine parts and are fully interchangeable. Many years will work also for those engines. The rebuilt carbs are ok, but one in ten are actually good. It would be best to rebuild your own since they are very simple.
Or I suggest having to install a C4 auto tranny and 5.0 V-8 engine. They have very easy kits with all mounts for it and then you can upgrade to 5 lug suspension with disc brakes pretty cheap.
You have alot of options as the 64 1/2 Stang has a huge following.
Just go on the web and enter "1964 1/2 straight 6 Ford Mustang carburetor" and you will have more information than you know what to do with. If you want to stay orginal it may be exspensive a "will fit" would be a lot cheaper.