Carburetor

So I need to replace my carburetor on my 71’ coupe 302. All I know is my old one is Holley 4B and it’s an aftermarket, also got told more than likely it was off an 80s F250. Anyway I have never bought a carburetor before and idk what to look for. I want to stick with Holley. Any advice helps 

Look on the choke air horn for the LIST#. If it's a Ford OE carb, it'll have a Ford part # stamped into the airhorn as well. 

https://www.ford-trucks.com/forums/attachment.php?attachmentid=139141&stc=1&d=1456862980

They are known as a 4180 model Holley, with certain emissions controls an annular primary boosters for better fuel mixture. A very good replacement for this would be the 80457SA with an electric choke. Holley has their factory refurbs on their site at a very good price, and it's in stock. The only downside is it doesn't have annular boosters. 

https://www.holley.com/products/fuel_systems/carburetors/factory_refurbished_carburetors/parts/FR-80457SA

Another option would be the Summit 600cfm, which does have annular boosters. I'm running the original Holley version with mechanical secondary version on my 71 and love it. If your engine is completely stock, the 500cfm would do well. 

https://www.summitracing.com/search/part-type/carburetors/brand/summit-racing?N=brand%3Asummit-racing%2Bcfm%3A600_500

Which one would be the best for my mustang? And thank you a ton helped me out a lot.

I agree with Hemikiller, the Summit 500cfm carburetor would be a good choice. The Summit carburetors combine the best elements of the Ford 4100 series carburetors with the Holley 4-barrels.