As Stainless says you really need a rule book. Your car could fit into 2 Categories (Classic or Modified) depending on minor changes. The engine swap puts you in a Gas class in either category.
Classic category (1928-1981) fits your year and body style and is more favorable to your body. Modified category is a body up to the current year and can be considerably sleeker putting you at a aero disadvantage. Kind of why the classic category was created. The difference is you have to run older technology i.e., no EFI unless it was OEM on the car. IOW, a carburetor or
mechanical FI. Even though the LS may be OEM EFI for that motor, the 69
body never had EFI so no classic category. Removing the EFI and adding the carb, or mechanical FI, lets you run Classic classes. If you want to run any computer managed, active feedback, EFI, then you stay in Modified category at an aero disadvantage.
Your rule book is your friend and will save you more money that you can imagine. But we don't race to save money do we!