I would Try to rebuild That 400 SB Chevy . I personally do not Like 383 Builds . Why Not Run a 350+30 over 355-375 HP 4-Bolt Main Build ? As for Your Truck Needing Cats ? I would say it would Need a Bit more then That for 1984 EPA Standards . Now its an Off Road Truck . It may Not need the EPA Testing in Your State any Longer. You will need to Find Out .