After Market Carb
…or if you don't want to mess with adaptors and stuff,
here is a new old stock '77 Super Six Carter BBD which will bolt directly to your intake. I don't recall whether your vehicle has an automatic or manual transmission; if automatic, see
this thread for kickdown options. You'd need to pick up a 2bbl air cleaner, and grab a № 1232
Electric choke kit, and…off you go!