Would premium gas harm your 1974 super beetle?

No. However if the car isn't pinging[1] it isn't needed.

[1] pinging (also called preignition, detonation, etc.) is when the air/fuel mix in the engine explodes before the spark plug sets it off. It often occurs under hard-strain situations, such as climbing a hill, heavy loads or fast acceleration. It sounds like someone rattling some pebbles in a coffee can. It is extremely bad for the engine and allowing it will damage the engine in short order.