Your ‘91 Avanti Convertible was really never meant to be a ‘fast’ car… An LS swap would, in my opinion, increase the value of your car… but you’ll spend quite a bit doing it, and the huge increase in power may cause other issues (adequate brakes?)
If the car were mine I’d probably overhaul the good old 305, if it’s getting tired, and enjoy the car for what it is…. Good luck! Ed