That year of engine, if the heads haven't been milled or higher compression pistons installed, should run fine on regular gas. Also depends on what the timing is at, and whether it's just cruising, pulling a load, or a bit of racing. If it's just an occasional driver, I guess I would put premium in it. Just better gas, better performance, and less prone to detonation. Hard to say for sure without knowing more about the engine internals.